ClickHouse/docs/en/operations/settings/settings.md

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Core Settings	2	/en/operations/settings/settings	2

Core Settings

additional_table_filters

An additional filter expression that is applied after reading from the specified table.

Default value: 0.

Example

INSERT INTO table_1 VALUES (1, 'a'), (2, 'bb'), (3, 'ccc'), (4, 'dddd');
SELECT * FROM table_1;

┌─x─┬─y────┐
│ 1 │ a    │
│ 2 │ bb   │
│ 3 │ ccc  │
│ 4 │ dddd │
└───┴──────┘

SELECT *
FROM table_1
SETTINGS additional_table_filters = {'table_1': 'x != 2'}

┌─x─┬─y────┐
│ 1 │ a    │
│ 3 │ ccc  │
│ 4 │ dddd │
└───┴──────┘

additional_result_filter

An additional filter expression to apply to the result of SELECT query. This setting is not applied to any subquery.

Default value: ''.

Example

INSERT INTO table_1 VALUES (1, 'a'), (2, 'bb'), (3, 'ccc'), (4, 'dddd');
SElECT * FROM table_1;

┌─x─┬─y────┐
│ 1 │ a    │
│ 2 │ bb   │
│ 3 │ ccc  │
│ 4 │ dddd │
└───┴──────┘

SELECT *
FROM table_1
SETTINGS additional_result_filter = 'x != 2'

┌─x─┬─y────┐
│ 1 │ a    │
│ 3 │ ccc  │
│ 4 │ dddd │
└───┴──────┘

allow_nondeterministic_mutations

User-level setting that allows mutations on replicated tables to make use of non-deterministic functions such as dictGet.

Given that, for example, dictionaries, can be out of sync across nodes, mutations that pull values from them are disallowed on replicated tables by default. Enabling this setting allows this behavior, making it the user's responsibility to ensure that the data used is in sync across all nodes.

Default value: 0.

Example

<profiles>
    <default>
        <allow_nondeterministic_mutations>1</allow_nondeterministic_mutations>

        <!-- ... -->
    </default>

    <!-- ... -->

</profiles>

mutations_execute_nondeterministic_on_initiator

If true constant nondeterministic functions (e.g. function now()) are executed on initiator and replaced to literals in UPDATE and DELETE queries. It helps to keep data in sync on replicas while executing mutations with constant nondeterministic functions. Default value: false.

mutations_execute_subqueries_on_initiator

If true scalar subqueries are executed on initiator and replaced to literals in UPDATE and DELETE queries. Default value: false.

mutations_max_literal_size_to_replace

The maximum size of serialized literal in bytes to replace in UPDATE and DELETE queries. Takes effect only if at least one the two settings above is enabled. Default value: 16384 (16 KiB).

distributed_product_mode

Changes the behaviour of distributed subqueries.

ClickHouse applies this setting when the query contains the product of distributed tables, i.e. when the query for a distributed table contains a non-GLOBAL subquery for the distributed table.

Restrictions:

• Only applied for IN and JOIN subqueries.
• Only if the FROM section uses a distributed table containing more than one shard.
• If the subquery concerns a distributed table containing more than one shard.
• Not used for a table-valued remote function.

Possible values:

• deny — Default value. Prohibits using these types of subqueries (returns the “Double-distributed in/JOIN subqueries is denied” exception).
• local — Replaces the database and table in the subquery with local ones for the destination server (shard), leaving the normal IN/JOIN.
• global — Replaces the IN/JOIN query with GLOBAL IN/GLOBAL JOIN.
• allow — Allows the use of these types of subqueries.

prefer_global_in_and_join

Enables the replacement of IN/JOIN operators with GLOBAL IN/GLOBAL JOIN.

Possible values:

• 0 — Disabled. IN/JOIN operators are not replaced with GLOBAL IN/GLOBAL JOIN.
• 1 — Enabled. IN/JOIN operators are replaced with GLOBAL IN/GLOBAL JOIN.

Default value: 0.

Usage

Although SET distributed_product_mode=global can change the queries behavior for the distributed tables, it's not suitable for local tables or tables from external resources. Here is when the prefer_global_in_and_join setting comes into play.

For example, we have query serving nodes that contain local tables, which are not suitable for distribution. We need to scatter their data on the fly during distributed processing with the GLOBAL keyword — GLOBAL IN/GLOBAL JOIN.

Another use case of prefer_global_in_and_join is accessing tables created by external engines. This setting helps to reduce the number of calls to external sources while joining such tables: only one call per query.

See also:

• Distributed subqueries for more information on how to use GLOBAL IN/GLOBAL JOIN

enable_optimize_predicate_expression

Turns on predicate pushdown in SELECT queries.

Predicate pushdown may significantly reduce network traffic for distributed queries.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 1.

Usage

Consider the following queries:

1. SELECT count() FROM test_table WHERE date = '2018-10-10'
2. SELECT count() FROM (SELECT * FROM test_table) WHERE date = '2018-10-10'

If enable_optimize_predicate_expression = 1, then the execution time of these queries is equal because ClickHouse applies WHERE to the subquery when processing it.

If enable_optimize_predicate_expression = 0, then the execution time of the second query is much longer because the WHERE clause applies to all the data after the subquery finishes.

fallback_to_stale_replicas_for_distributed_queries

Forces a query to an out-of-date replica if updated data is not available. See Replication.

ClickHouse selects the most relevant from the outdated replicas of the table.

Used when performing SELECT from a distributed table that points to replicated tables.

By default, 1 (enabled).

force_index_by_date

Disables query execution if the index can’t be used by date.

Works with tables in the MergeTree family.

If force_index_by_date=1, ClickHouse checks whether the query has a date key condition that can be used for restricting data ranges. If there is no suitable condition, it throws an exception. However, it does not check whether the condition reduces the amount of data to read. For example, the condition Date != ' 2000-01-01 ' is acceptable even when it matches all the data in the table (i.e., running the query requires a full scan). For more information about ranges of data in MergeTree tables, see MergeTree.

force_primary_key

Disables query execution if indexing by the primary key is not possible.

Works with tables in the MergeTree family.

If force_primary_key=1, ClickHouse checks to see if the query has a primary key condition that can be used for restricting data ranges. If there is no suitable condition, it throws an exception. However, it does not check whether the condition reduces the amount of data to read. For more information about data ranges in MergeTree tables, see MergeTree.

use_skip_indexes

Use data skipping indexes during query execution.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 1.

force_data_skipping_indices

Disables query execution if passed data skipping indices wasn't used.

Consider the following example:

CREATE TABLE data
(
    key Int,
    d1 Int,
    d1_null Nullable(Int),
    INDEX d1_idx d1 TYPE minmax GRANULARITY 1,
    INDEX d1_null_idx assumeNotNull(d1_null) TYPE minmax GRANULARITY 1
)
Engine=MergeTree()
ORDER BY key;

SELECT * FROM data_01515;
SELECT * FROM data_01515 SETTINGS force_data_skipping_indices=''; -- query will produce CANNOT_PARSE_TEXT error.
SELECT * FROM data_01515 SETTINGS force_data_skipping_indices='d1_idx'; -- query will produce INDEX_NOT_USED error.
SELECT * FROM data_01515 WHERE d1 = 0 SETTINGS force_data_skipping_indices='d1_idx'; -- Ok.
SELECT * FROM data_01515 WHERE d1 = 0 SETTINGS force_data_skipping_indices='`d1_idx`'; -- Ok (example of full featured parser).
SELECT * FROM data_01515 WHERE d1 = 0 SETTINGS force_data_skipping_indices='`d1_idx`, d1_null_idx'; -- query will produce INDEX_NOT_USED error, since d1_null_idx is not used.
SELECT * FROM data_01515 WHERE d1 = 0 AND assumeNotNull(d1_null) = 0 SETTINGS force_data_skipping_indices='`d1_idx`, d1_null_idx'; -- Ok.

ignore_data_skipping_indices

Ignores the skipping indexes specified if used by the query.

Consider the following example:

CREATE TABLE data
(
    key Int,
    x Int,
    y Int,
    INDEX x_idx x TYPE minmax GRANULARITY 1,
    INDEX y_idx y TYPE minmax GRANULARITY 1,
    INDEX xy_idx (x,y) TYPE minmax GRANULARITY 1
)
Engine=MergeTree()
ORDER BY key;

INSERT INTO data VALUES (1, 2, 3);

SELECT * FROM data;
SELECT * FROM data SETTINGS ignore_data_skipping_indices=''; -- query will produce CANNOT_PARSE_TEXT error.
SELECT * FROM data SETTINGS ignore_data_skipping_indices='x_idx'; -- Ok.
SELECT * FROM data SETTINGS ignore_data_skipping_indices='na_idx'; -- Ok.

SELECT * FROM data WHERE x = 1 AND y = 1 SETTINGS ignore_data_skipping_indices='xy_idx',force_data_skipping_indices='xy_idx' ; -- query will produce INDEX_NOT_USED error, since xy_idx is explictly ignored.
SELECT * FROM data WHERE x = 1 AND y = 2 SETTINGS ignore_data_skipping_indices='xy_idx';

The query without ignoring any indexes:

EXPLAIN indexes = 1 SELECT * FROM data WHERE x = 1 AND y = 2;

Expression ((Projection + Before ORDER BY))
  Filter (WHERE)
    ReadFromMergeTree (default.data)
    Indexes:
      PrimaryKey
        Condition: true
        Parts: 1/1
        Granules: 1/1
      Skip
        Name: x_idx
        Description: minmax GRANULARITY 1
        Parts: 0/1
        Granules: 0/1
      Skip
        Name: y_idx
        Description: minmax GRANULARITY 1
        Parts: 0/0
        Granules: 0/0
      Skip
        Name: xy_idx
        Description: minmax GRANULARITY 1
        Parts: 0/0
        Granules: 0/0

Ignoring the xy_idx index:

EXPLAIN indexes = 1 SELECT * FROM data WHERE x = 1 AND y = 2 SETTINGS ignore_data_skipping_indices='xy_idx';

Expression ((Projection + Before ORDER BY))
  Filter (WHERE)
    ReadFromMergeTree (default.data)
    Indexes:
      PrimaryKey
        Condition: true
        Parts: 1/1
        Granules: 1/1
      Skip
        Name: x_idx
        Description: minmax GRANULARITY 1
        Parts: 0/1
        Granules: 0/1
      Skip
        Name: y_idx
        Description: minmax GRANULARITY 1
        Parts: 0/0
        Granules: 0/0

Works with tables in the MergeTree family.

convert_query_to_cnf

When set to true, a SELECT query will be converted to conjuctive normal form (CNF). There are scenarios where rewriting a query in CNF may execute faster (view this Github issue for an explanation).

For example, notice how the following SELECT query is not modified (the default behavior):

EXPLAIN SYNTAX
SELECT *
FROM
(
    SELECT number AS x
    FROM numbers(20)
) AS a
WHERE ((x >= 1) AND (x <= 5)) OR ((x >= 10) AND (x <= 15))
SETTINGS convert_query_to_cnf = false;

The result is:

┌─explain────────────────────────────────────────────────────────┐
│ SELECT x                                                       │
│ FROM                                                           │
│ (                                                              │
│     SELECT number AS x                                         │
│     FROM numbers(20)                                           │
│     WHERE ((x >= 1) AND (x <= 5)) OR ((x >= 10) AND (x <= 15)) │
│ ) AS a                                                         │
│ WHERE ((x >= 1) AND (x <= 5)) OR ((x >= 10) AND (x <= 15))     │
│ SETTINGS convert_query_to_cnf = 0                              │
└────────────────────────────────────────────────────────────────┘

Let's set convert_query_to_cnf to true and see what changes:

EXPLAIN SYNTAX
SELECT *
FROM
(
    SELECT number AS x
    FROM numbers(20)
) AS a
WHERE ((x >= 1) AND (x <= 5)) OR ((x >= 10) AND (x <= 15))
SETTINGS convert_query_to_cnf = true;

Notice the WHERE clause is rewritten in CNF, but the result set is the identical - the Boolean logic is unchanged:

┌─explain───────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ SELECT x                                                                                                              │
│ FROM                                                                                                                  │
│ (                                                                                                                     │
│     SELECT number AS x                                                                                                │
│     FROM numbers(20)                                                                                                  │
│     WHERE ((x <= 15) OR (x <= 5)) AND ((x <= 15) OR (x >= 1)) AND ((x >= 10) OR (x <= 5)) AND ((x >= 10) OR (x >= 1)) │
│ ) AS a                                                                                                                │
│ WHERE ((x >= 10) OR (x >= 1)) AND ((x >= 10) OR (x <= 5)) AND ((x <= 15) OR (x >= 1)) AND ((x <= 15) OR (x <= 5))     │
│ SETTINGS convert_query_to_cnf = 1                                                                                     │
└───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘

Possible values: true, false

Default value: false

fsync_metadata

Enables or disables fsync when writing .sql files. Enabled by default.

It makes sense to disable it if the server has millions of tiny tables that are constantly being created and destroyed.

function_range_max_elements_in_block

Sets the safety threshold for data volume generated by function range. Defines the maximum number of values generated by function per block of data (sum of array sizes for every row in a block).

Possible values:

• Positive integer.

Default value: 500,000,000.

See Also

• max_block_size
• min_insert_block_size_rows

enable_http_compression

Enables or disables data compression in the response to an HTTP request.

For more information, read the HTTP interface description.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 0.

http_zlib_compression_level

Sets the level of data compression in the response to an HTTP request if enable_http_compression = 1.

Possible values: Numbers from 1 to 9.

Default value: 3.

http_native_compression_disable_checksumming_on_decompress

Enables or disables checksum verification when decompressing the HTTP POST data from the client. Used only for ClickHouse native compression format (not used with gzip or deflate).

For more information, read the HTTP interface description.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 0.

http_max_uri_size

Sets the maximum URI length of an HTTP request.

Possible values:

• Positive integer.

Default value: 1048576.

http_make_head_request

The http_make_head_request setting allows the execution of a HEAD request while reading data from HTTP to retrieve information about the file to be read, such as its size. Since it's enabled by default, it may be desirable to disable this setting in cases where the server does not support HEAD requests.

Default value: true.

table_function_remote_max_addresses

Sets the maximum number of addresses generated from patterns for the remote function.

Possible values:

• Positive integer.

Default value: 1000.

glob_expansion_max_elements

Sets the maximum number of addresses generated from patterns for external storages and table functions (like url) except the remote function.

Possible values:

• Positive integer.

Default value: 1000.

send_progress_in_http_headers

Enables or disables X-ClickHouse-Progress HTTP response headers in clickhouse-server responses.

For more information, read the HTTP interface description.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 0.

max_http_get_redirects

Limits the maximum number of HTTP GET redirect hops for URL-engine tables. The setting applies to both types of tables: those created by the CREATE TABLE query and by the url table function.

Possible values:

• Any positive integer number of hops.
• 0 — No hops allowed.

Default value: 0.

Cloud default value: 10.

insert_null_as_default

Enables or disables the insertion of default values instead of NULL into columns with not nullable data type. If column type is not nullable and this setting is disabled, then inserting NULL causes an exception. If column type is nullable, then NULL values are inserted as is, regardless of this setting.

This setting is applicable to INSERT ... SELECT queries. Note that SELECT subqueries may be concatenated with UNION ALL clause.

Possible values:

• 0 — Inserting NULL into a not nullable column causes an exception.
• 1 — Default column value is inserted instead of NULL.

Default value: 1.

join_default_strictness

Sets default strictness for JOIN clauses.

Possible values:

• ALL — If the right table has several matching rows, ClickHouse creates a Cartesian product from matching rows. This is the normal JOIN behaviour from standard SQL.
• ANY — If the right table has several matching rows, only the first one found is joined. If the right table has only one matching row, the results of ANY and ALL are the same.
• ASOF — For joining sequences with an uncertain match.
• Empty string — If ALL or ANY is not specified in the query, ClickHouse throws an exception.

Default value: ALL.

join_algorithm

Specifies which JOIN algorithm is used.

Several algorithms can be specified, and an available one would be chosen for a particular query based on kind/strictness and table engine.

Possible values:

• default

This is the equivalent of hash or direct, if possible (same as direct,hash)

• grace_hash

Grace hash join is used. Grace hash provides an algorithm option that provides performant complex joins while limiting memory use.

The first phase of a grace join reads the right table and splits it into N buckets depending on the hash value of key columns (initially, N is grace_hash_join_initial_buckets). This is done in a way to ensure that each bucket can be processed independently. Rows from the first bucket are added to an in-memory hash table while the others are saved to disk. If the hash table grows beyond the memory limit (e.g., as set by max_bytes_in_join), the number of buckets is increased and the assigned bucket for each row. Any rows which don’t belong to the current bucket are flushed and reassigned.

Supports INNER/LEFT/RIGHT/FULL ALL/ANY JOIN.

• hash

Hash join algorithm is used. The most generic implementation that supports all combinations of kind and strictness and multiple join keys that are combined with OR in the JOIN ON section.

• parallel_hash

A variation of hash join that splits the data into buckets and builds several hashtables instead of one concurrently to speed up this process.

When using the hash algorithm, the right part of JOIN is uploaded into RAM.

• partial_merge

A variation of the sort-merge algorithm, where only the right table is fully sorted.

The RIGHT JOIN and FULL JOIN are supported only with ALL strictness (SEMI, ANTI, ANY, and ASOF are not supported).

When using the partial_merge algorithm, ClickHouse sorts the data and dumps it to the disk. The partial_merge algorithm in ClickHouse differs slightly from the classic realization. First, ClickHouse sorts the right table by joining keys in blocks and creates a min-max index for sorted blocks. Then it sorts parts of the left table by the join key and joins them over the right table. The min-max index is also used to skip unneeded right table blocks.

• direct

This algorithm can be applied when the storage for the right table supports key-value requests.

The direct algorithm performs a lookup in the right table using rows from the left table as keys. It's supported only by special storage such as Dictionary or EmbeddedRocksDB and only the LEFT and INNER JOINs.

• auto

When set to auto, hash join is tried first, and the algorithm is switched on the fly to another algorithm if the memory limit is violated.

• full_sorting_merge

Sort-merge algorithm with full sorting joined tables before joining.

• prefer_partial_merge

ClickHouse always tries to use partial_merge join if possible, otherwise, it uses hash. Deprecated, same as partial_merge,hash.

join_any_take_last_row

Changes the behaviour of join operations with ANY strictness.

:::note This setting applies only for JOIN operations with Join engine tables. :::

Possible values:

• 0 — If the right table has more than one matching row, only the first one found is joined.
• 1 — If the right table has more than one matching row, only the last one found is joined.

Default value: 0.

See also:

• JOIN clause
• Join table engine
• join_default_strictness

join_use_nulls

Sets the type of JOIN behaviour. When merging tables, empty cells may appear. ClickHouse fills them differently based on this setting.

Possible values:

• 0 — The empty cells are filled with the default value of the corresponding field type.
• 1 — JOIN behaves the same way as in standard SQL. The type of the corresponding field is converted to Nullable, and empty cells are filled with NULL.

Default value: 0.

group_by_use_nulls

Changes the way the GROUP BY clause treats the types of aggregation keys. When the ROLLUP, CUBE, or GROUPING SETS specifiers are used, some aggregation keys may not be used to produce some result rows. Columns for these keys are filled with either default value or NULL in corresponding rows depending on this setting.

Possible values:

• 0 — The default value for the aggregation key type is used to produce missing values.
• 1 — ClickHouse executes GROUP BY the same way as the SQL standard says. The types of aggregation keys are converted to Nullable. Columns for corresponding aggregation keys are filled with NULL for rows that didn't use it.

Default value: 0.

See also:

• GROUP BY clause

partial_merge_join_optimizations

Disables optimizations in partial merge join algorithm for JOIN queries.

By default, this setting enables improvements that could lead to wrong results. If you see suspicious results in your queries, disable optimizations by this setting. Optimizations can be different in different versions of the ClickHouse server.

Possible values:

• 0 — Optimizations disabled.
• 1 — Optimizations enabled.

Default value: 1.

partial_merge_join_rows_in_right_blocks

Limits sizes of right-hand join data blocks in partial merge join algorithm for JOIN queries.

ClickHouse server:

1. Splits right-hand join data into blocks with up to the specified number of rows.
2. Indexes each block with its minimum and maximum values.
3. Unloads prepared blocks to disk if it is possible.

Possible values:

• Any positive integer. Recommended range of values: [1000, 100000].

Default value: 65536.

join_on_disk_max_files_to_merge

Limits the number of files allowed for parallel sorting in MergeJoin operations when they are executed on disk.

The bigger the value of the setting, the more RAM is used and the less disk I/O is needed.

Possible values:

• Any positive integer, starting from 2.

Default value: 64.

any_join_distinct_right_table_keys

Enables legacy ClickHouse server behaviour in ANY INNER|LEFT JOIN operations.

:::note Use this setting only for backward compatibility if your use cases depend on legacy JOIN behaviour. :::

When the legacy behaviour is enabled:

• Results of t1 ANY LEFT JOIN t2 and t2 ANY RIGHT JOIN t1 operations are not equal because ClickHouse uses the logic with many-to-one left-to-right table keys mapping.
• Results of ANY INNER JOIN operations contain all rows from the left table like the SEMI LEFT JOIN operations do.

When the legacy behaviour is disabled:

• Results of t1 ANY LEFT JOIN t2 and t2 ANY RIGHT JOIN t1 operations are equal because ClickHouse uses the logic which provides one-to-many keys mapping in ANY RIGHT JOIN operations.
• Results of ANY INNER JOIN operations contain one row per key from both the left and right tables.

Possible values:

• 0 — Legacy behaviour is disabled.
• 1 — Legacy behaviour is enabled.

Default value: 0.

See also:

• JOIN strictness

max_rows_in_set_to_optimize_join

Maximal size of the set to filter joined tables by each other's row sets before joining.

Possible values:

• 0 — Disable.
• Any positive integer.

Default value: 100000.

temporary_files_codec

Sets compression codec for temporary files used in sorting and joining operations on disk.

Possible values:

• LZ4 — LZ4 compression is applied.
• NONE — No compression is applied.

Default value: LZ4.

max_block_size

In ClickHouse, data is processed by blocks, which are sets of column parts. The internal processing cycles for a single block are efficient but there are noticeable costs when processing each block.

The max_block_size setting indicates the recommended maximum number of rows to include in a single block when loading data from tables. Blocks the size of max_block_size are not always loaded from the table: if ClickHouse determines that less data needs to be retrieved, a smaller block is processed.

The block size should not be too small to avoid noticeable costs when processing each block. It should also not be too large to ensure that queries with a LIMIT clause execute quickly after processing the first block. When setting max_block_size, the goal should be to avoid consuming too much memory when extracting a large number of columns in multiple threads and to preserve at least some cache locality.

Default value: 65,409

preferred_block_size_bytes

Used for the same purpose as max_block_size, but it sets the recommended block size in bytes by adapting it to the number of rows in the block. However, the block size cannot be more than max_block_size rows. By default: 1,000,000. It only works when reading from MergeTree engines.

max_concurrent_queries_for_user

The maximum number of simultaneously processed queries per user.

Possible values:

• Positive integer.
• 0 — No limit.

Default value: 0.

Example

<max_concurrent_queries_for_user>5</max_concurrent_queries_for_user>

max_concurrent_queries_for_all_users

Throw exception if the value of this setting is less or equal than the current number of simultaneously processed queries.

Example: max_concurrent_queries_for_all_users can be set to 99 for all users and database administrator can set it to 100 for itself to run queries for investigation even when the server is overloaded.

Modifying the setting for one query or user does not affect other queries.

Possible values:

• Positive integer.
• 0 — No limit.

Default value: 0.

Example

<max_concurrent_queries_for_all_users>99</max_concurrent_queries_for_all_users>

See Also

• max_concurrent_queries

merge_tree_min_rows_for_concurrent_read

If the number of rows to be read from a file of a MergeTree table exceeds merge_tree_min_rows_for_concurrent_read then ClickHouse tries to perform a concurrent reading from this file on several threads.

Possible values:

• Positive integer.

Default value: 163840.

merge_tree_min_rows_for_concurrent_read_for_remote_filesystem

The minimum number of lines to read from one file before the MergeTree engine can parallelize reading, when reading from remote filesystem.

Possible values:

• Positive integer.

Default value: 163840.

merge_tree_min_bytes_for_concurrent_read

If the number of bytes to read from one file of a MergeTree-engine table exceeds merge_tree_min_bytes_for_concurrent_read, then ClickHouse tries to concurrently read from this file in several threads.

Possible value:

• Positive integer.

Default value: 251658240.

merge_tree_min_bytes_for_concurrent_read_for_remote_filesystem

The minimum number of bytes to read from one file before MergeTree engine can parallelize reading, when reading from remote filesystem.

Possible values:

• Positive integer.

Default value: 251658240.

merge_tree_min_rows_for_seek

If the distance between two data blocks to be read in one file is less than merge_tree_min_rows_for_seek rows, then ClickHouse does not seek through the file but reads the data sequentially.

Possible values:

• Any positive integer.

Default value: 0.

merge_tree_min_bytes_for_seek

If the distance between two data blocks to be read in one file is less than merge_tree_min_bytes_for_seek bytes, then ClickHouse sequentially reads a range of file that contains both blocks, thus avoiding extra seek.

Possible values:

• Any positive integer.

Default value: 0.

merge_tree_coarse_index_granularity

When searching for data, ClickHouse checks the data marks in the index file. If ClickHouse finds that required keys are in some range, it divides this range into merge_tree_coarse_index_granularity subranges and searches the required keys there recursively.

Possible values:

• Any positive even integer.

Default value: 8.

merge_tree_max_rows_to_use_cache

If ClickHouse should read more than merge_tree_max_rows_to_use_cache rows in one query, it does not use the cache of uncompressed blocks.

The cache of uncompressed blocks stores data extracted for queries. ClickHouse uses this cache to speed up responses to repeated small queries. This setting protects the cache from trashing by queries that read a large amount of data. The uncompressed_cache_size server setting defines the size of the cache of uncompressed blocks.

Possible values:

• Any positive integer.

Default value: 128 ✕ 8192.

merge_tree_max_bytes_to_use_cache

If ClickHouse should read more than merge_tree_max_bytes_to_use_cache bytes in one query, it does not use the cache of uncompressed blocks.

The cache of uncompressed blocks stores data extracted for queries. ClickHouse uses this cache to speed up responses to repeated small queries. This setting protects the cache from trashing by queries that read a large amount of data. The uncompressed_cache_size server setting defines the size of the cache of uncompressed blocks.

Possible values:

• Any positive integer.

Default value: 2013265920.

min_bytes_to_use_direct_io

The minimum data volume required for using direct I/O access to the storage disk.

ClickHouse uses this setting when reading data from tables. If the total storage volume of all the data to be read exceeds min_bytes_to_use_direct_io bytes, then ClickHouse reads the data from the storage disk with the O_DIRECT option.

Possible values:

• 0 — Direct I/O is disabled.
• Positive integer.

Default value: 0.

network_compression_method

Sets the method of data compression that is used for communication between servers and between server and clickhouse-client.

Possible values:

• LZ4 — sets LZ4 compression method.
• ZSTD — sets ZSTD compression method.

Default value: LZ4.

See Also

• network_zstd_compression_level

network_zstd_compression_level

Adjusts the level of ZSTD compression. Used only when network_compression_method is set to ZSTD.

Possible values:

• Positive integer from 1 to 15.

Default value: 1.

log_queries

Setting up query logging.

Queries sent to ClickHouse with this setup are logged according to the rules in the query_log server configuration parameter.

Example:

log_queries=1

log_queries_min_query_duration_ms

If enabled (non-zero), queries faster than the value of this setting will not be logged (you can think about this as a long_query_time for MySQL Slow Query Log), and this basically means that you will not find them in the following tables:

• system.query_log
• system.query_thread_log

Only the queries with the following type will get to the log:

• QUERY_FINISH

• EXCEPTION_WHILE_PROCESSING

• Type: milliseconds

• Default value: 0 (any query)

log_queries_min_type

query_log minimal type to log.

Possible values:

• QUERY_START (=1)
• QUERY_FINISH (=2)
• EXCEPTION_BEFORE_START (=3)
• EXCEPTION_WHILE_PROCESSING (=4)

Default value: QUERY_START.

Can be used to limit which entities will go to query_log, say you are interested only in errors, then you can use EXCEPTION_WHILE_PROCESSING:

log_queries_min_type='EXCEPTION_WHILE_PROCESSING'

log_query_threads

Setting up query threads logging.

Query threads log into the system.query_thread_log table. This setting has effect only when log_queries is true. Queries’ threads run by ClickHouse with this setup are logged according to the rules in the query_thread_log server configuration parameter.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 1.

Example

log_query_threads=1

log_query_views

Setting up query views logging.

When a query run by ClickHouse with this setting enabled has associated views (materialized or live views), they are logged in the query_views_log server configuration parameter.

Example:

log_query_views=1

log_formatted_queries

Allows to log formatted queries to the system.query_log system table (populates formatted_query column in the system.query_log).

Possible values:

• 0 — Formatted queries are not logged in the system table.
• 1 — Formatted queries are logged in the system table.

Default value: 0.

log_comment

Specifies the value for the log_comment field of the system.query_log table and comment text for the server log.

It can be used to improve the readability of server logs. Additionally, it helps to select queries related to the test from the system.query_log after running clickhouse-test.

Possible values:

• Any string no longer than max_query_size. If the max_query_size is exceeded, the server throws an exception.

Default value: empty string.

Example

Query:

SET log_comment = 'log_comment test', log_queries = 1;
SELECT 1;
SYSTEM FLUSH LOGS;
SELECT type, query FROM system.query_log WHERE log_comment = 'log_comment test' AND event_date >= yesterday() ORDER BY event_time DESC LIMIT 2;

Result:

┌─type────────┬─query─────┐
│ QueryStart  │ SELECT 1; │
│ QueryFinish │ SELECT 1; │
└─────────────┴───────────┘

log_processors_profiles

Write time that processor spent during execution/waiting for data to system.processors_profile_log table.

See also:

• system.processors_profile_log
• EXPLAIN PIPELINE

max_insert_block_size

The size of blocks (in a count of rows) to form for insertion into a table. This setting only applies in cases when the server forms the blocks. For example, for an INSERT via the HTTP interface, the server parses the data format and forms blocks of the specified size. But when using clickhouse-client, the client parses the data itself, and the ‘max_insert_block_size’ setting on the server does not affect the size of the inserted blocks. The setting also does not have a purpose when using INSERT SELECT, since data is inserted using the same blocks that are formed after SELECT.

Default value: 1,048,576.

The default is slightly more than max_block_size. The reason for this is that certain table engines (*MergeTree) form a data part on the disk for each inserted block, which is a fairly large entity. Similarly, *MergeTree tables sort data during insertion, and a large enough block size allow sorting more data in RAM.

min_insert_block_size_rows

Sets the minimum number of rows in the block that can be inserted into a table by an INSERT query. Smaller-sized blocks are squashed into bigger ones.

Possible values:

• Positive integer.
• 0 — Squashing disabled.

Default value: 1048576.

min_insert_block_size_bytes

Sets the minimum number of bytes in the block which can be inserted into a table by an INSERT query. Smaller-sized blocks are squashed into bigger ones.

Possible values:

• Positive integer.
• 0 — Squashing disabled.

Default value: 268435456.

max_replica_delay_for_distributed_queries

Disables lagging replicas for distributed queries. See Replication.

Sets the time in seconds. If a replica's lag is greater than or equal to the set value, this replica is not used.

Possible values:

• Positive integer.
• 0 — Replica lags are not checked.

To prevent the use of any replica with a non-zero lag, set this parameter to 1.

Default value: 300.

Used when performing SELECT from a distributed table that points to replicated tables.

max_threads

The maximum number of query processing threads, excluding threads for retrieving data from remote servers (see the ‘max_distributed_connections’ parameter).

This parameter applies to threads that perform the same stages of the query processing pipeline in parallel. For example, when reading from a table, if it is possible to evaluate expressions with functions, filter with WHERE and pre-aggregate for GROUP BY in parallel using at least ‘max_threads’ number of threads, then ‘max_threads’ are used.

Default value: the number of physical CPU cores.

For queries that are completed quickly because of a LIMIT, you can set a lower ‘max_threads’. For example, if the necessary number of entries are located in every block and max_threads = 8, then 8 blocks are retrieved, although it would have been enough to read just one.

The smaller the max_threads value, the less memory is consumed.

max_insert_threads

The maximum number of threads to execute the INSERT SELECT query.

Possible values:

• 0 (or 1) — INSERT SELECT no parallel execution.
• Positive integer. Bigger than 1.

Default value: 0.

Cloud default value: from 2 to 4, depending on the service size.

Parallel INSERT SELECT has effect only if the SELECT part is executed in parallel, see max_threads setting. Higher values will lead to higher memory usage.

max_compress_block_size

The maximum size of blocks of uncompressed data before compressing for writing to a table. By default, 1,048,576 (1 MiB). Specifying a smaller block size generally leads to slightly reduced compression ratio, the compression and decompression speed increases slightly due to cache locality, and memory consumption is reduced.

:::note This is an expert-level setting, and you shouldn't change it if you're just getting started with ClickHouse. :::

Don’t confuse blocks for compression (a chunk of memory consisting of bytes) with blocks for query processing (a set of rows from a table).

min_compress_block_size

For MergeTree tables. In order to reduce latency when processing queries, a block is compressed when writing the next mark if its size is at least min_compress_block_size. By default, 65,536.

The actual size of the block, if the uncompressed data is less than max_compress_block_size, is no less than this value and no less than the volume of data for one mark.

Let’s look at an example. Assume that index_granularity was set to 8192 during table creation.

We are writing a UInt32-type column (4 bytes per value). When writing 8192 rows, the total will be 32 KB of data. Since min_compress_block_size = 65,536, a compressed block will be formed for every two marks.

We are writing a URL column with the String type (average size of 60 bytes per value). When writing 8192 rows, the average will be slightly less than 500 KB of data. Since this is more than 65,536, a compressed block will be formed for each mark. In this case, when reading data from the disk in the range of a single mark, extra data won’t be decompressed.

:::note This is an expert-level setting, and you shouldn't change it if you're just getting started with ClickHouse. :::

max_query_size

The maximum number of bytes of a query string parsed by the SQL parser. Data in the VALUES clause of INSERT queries is processed by a separate stream parser (that consumes O(1) RAM) and not affected by this restriction.

Default value: 262144 (= 256 KiB).

max_parser_depth

Limits maximum recursion depth in the recursive descent parser. Allows controlling the stack size.

Possible values:

• Positive integer.
• 0 — Recursion depth is unlimited.

Default value: 1000.

interactive_delay

The interval in microseconds for checking whether request execution has been canceled and sending the progress.

Default value: 100,000 (checks for cancelling and sends the progress ten times per second).

idle_connection_timeout

Timeout to close idle TCP connections after specified number of seconds.

Possible values:

• Positive integer (0 - close immediately, after 0 seconds).

Default value: 3600.

connect_timeout, receive_timeout, send_timeout

Timeouts in seconds on the socket used for communicating with the client.

Default value: 10, 300, 300.

handshake_timeout_ms

Timeout in milliseconds for receiving Hello packet from replicas during handshake.

Default value: 10000.

cancel_http_readonly_queries_on_client_close

Cancels HTTP read-only queries (e.g. SELECT) when a client closes the connection without waiting for the response.

Default value: 0.

Cloud default value: 1.

poll_interval

Lock in a wait loop for the specified number of seconds.

Default value: 10.

max_distributed_connections

The maximum number of simultaneous connections with remote servers for distributed processing of a single query to a single Distributed table. We recommend setting a value no less than the number of servers in the cluster.

Default value: 1024.

The following parameters are only used when creating Distributed tables (and when launching a server), so there is no reason to change them at runtime.

distributed_connections_pool_size

The maximum number of simultaneous connections with remote servers for distributed processing of all queries to a single Distributed table. We recommend setting a value no less than the number of servers in the cluster.

Default value: 1024.

max_distributed_depth

Limits the maximum depth of recursive queries for Distributed tables.

If the value is exceeded, the server throws an exception.

Possible values:

• Positive integer.
• 0 — Unlimited depth.

Default value: 5.

max_replicated_fetches_network_bandwidth_for_server

Limits the maximum speed of data exchange over the network in bytes per second for replicated fetches for the server. Only has meaning at server startup. You can also limit the speed for a particular table with max_replicated_fetches_network_bandwidth setting.

The setting isn't followed perfectly accurately.

Possible values:

• Positive integer.
• 0 — Unlimited.

Default value: 0.

Usage

Could be used for throttling speed when replicating the data to add or replace new nodes.

:::note 60000000 bytes/s approximately corresponds to 457 Mbps (60000000 / 1024 / 1024 * 8). :::

max_replicated_sends_network_bandwidth_for_server

Limits the maximum speed of data exchange over the network in bytes per second for replicated sends for the server. Only has meaning at server startup. You can also limit the speed for a particular table with max_replicated_sends_network_bandwidth setting.

The setting isn't followed perfectly accurately.

Possible values:

• Positive integer.
• 0 — Unlimited.

Default value: 0.

Usage

Could be used for throttling speed when replicating the data to add or replace new nodes.

:::note 60000000 bytes/s approximately corresponds to 457 Mbps (60000000 / 1024 / 1024 * 8). :::

connect_timeout_with_failover_ms

The timeout in milliseconds for connecting to a remote server for a Distributed table engine, if the ‘shard’ and ‘replica’ sections are used in the cluster definition. If unsuccessful, several attempts are made to connect to various replicas.

Default value: 1000.

connect_timeout_with_failover_secure_ms

Connection timeout for selecting first healthy replica (for secure connections)

Default value: 1000.

connection_pool_max_wait_ms

The wait time in milliseconds for a connection when the connection pool is full.

Possible values:

• Positive integer.
• 0 — Infinite timeout.

Default value: 0.

connections_with_failover_max_tries

The maximum number of connection attempts with each replica for the Distributed table engine.

Default value: 3.

extremes

Whether to count extreme values (the minimums and maximums in columns of a query result). Accepts 0 or 1. By default, 0 (disabled). For more information, see the section “Extreme values”.

kafka_max_wait_ms

The wait time in milliseconds for reading messages from Kafka before retry.

Possible values:

• Positive integer.
• 0 — Infinite timeout.

Default value: 5000.

See also:

• Apache Kafka

kafka_disable_num_consumers_limit

Disable limit on kafka_num_consumers that depends on the number of available CPU cores.

Default value: false.

postgresql_connection_pool_size

Connection pool size for PostgreSQL table engine and database engine.

Default value: 16

postgresql_connection_pool_wait_timeout

Connection pool push/pop timeout on empty pool for PostgreSQL table engine and database engine. By default it will block on empty pool.

Default value: 5000

postgresql_connection_pool_auto_close_connection

Close connection before returning connection to the pool.

Default value: true.

odbc_bridge_connection_pool_size

Connection pool size for each connection settings string in ODBC bridge.

Default value: 16

odbc_bridge_use_connection_pooling

Use connection pooling in ODBC bridge. If set to false, a new connection is created every time.

Default value: true

use_uncompressed_cache

Whether to use a cache of uncompressed blocks. Accepts 0 or 1. By default, 0 (disabled). Using the uncompressed cache (only for tables in the MergeTree family) can significantly reduce latency and increase throughput when working with a large number of short queries. Enable this setting for users who send frequent short requests. Also pay attention to the uncompressed_cache_size configuration parameter (only set in the config file) – the size of uncompressed cache blocks. By default, it is 8 GiB. The uncompressed cache is filled in as needed and the least-used data is automatically deleted.

For queries that read at least a somewhat large volume of data (one million rows or more), the uncompressed cache is disabled automatically to save space for truly small queries. This means that you can keep the ‘use_uncompressed_cache’ setting always set to 1.

replace_running_query

When using the HTTP interface, the ‘query_id’ parameter can be passed. This is any string that serves as the query identifier. If a query from the same user with the same ‘query_id’ already exists at this time, the behaviour depends on the ‘replace_running_query’ parameter.

0 (default) – Throw an exception (do not allow the query to run if a query with the same ‘query_id’ is already running).

1 – Cancel the old query and start running the new one.

Set this parameter to 1 for implementing suggestions for segmentation conditions. After entering the next character, if the old query hasn’t finished yet, it should be cancelled.

replace_running_query_max_wait_ms

The wait time for running the query with the same query_id to finish, when the replace_running_query setting is active.

Possible values:

• Positive integer.
• 0 — Throwing an exception that does not allow to run a new query if the server already executes a query with the same query_id.

Default value: 5000.

stream_flush_interval_ms

Works for tables with streaming in the case of a timeout, or when a thread generates max_insert_block_size rows.

The default value is 7500.

The smaller the value, the more often data is flushed into the table. Setting the value too low leads to poor performance.

stream_poll_timeout_ms

Timeout for polling data from/to streaming storages.

Default value: 500.

load_balancing

Specifies the algorithm of replicas selection that is used for distributed query processing.

ClickHouse supports the following algorithms of choosing replicas:

• Random (by default)
• Nearest hostname
• Hostname levenshtein distance
• In order
• First or random
• Round robin

See also:

• distributed_replica_max_ignored_errors

Random (by Default)

load_balancing = random

The number of errors is counted for each replica. The query is sent to the replica with the fewest errors, and if there are several of these, to anyone of them. Disadvantages: Server proximity is not accounted for; if the replicas have different data, you will also get different data.

Nearest Hostname

load_balancing = nearest_hostname

The number of errors is counted for each replica. Every 5 minutes, the number of errors is integrally divided by 2. Thus, the number of errors is calculated for a recent time with exponential smoothing. If there is one replica with a minimal number of errors (i.e. errors occurred recently on the other replicas), the query is sent to it. If there are multiple replicas with the same minimal number of errors, the query is sent to the replica with a hostname that is most similar to the server’s hostname in the config file (for the number of different characters in identical positions, up to the minimum length of both hostnames).

For instance, example01-01-1 and example01-01-2 are different in one position, while example01-01-1 and example01-02-2 differ in two places. This method might seem primitive, but it does not require external data about network topology, and it does not compare IP addresses, which would be complicated for our IPv6 addresses.

Thus, if there are equivalent replicas, the closest one by name is preferred. We can also assume that when sending a query to the same server, in the absence of failures, a distributed query will also go to the same servers. So even if different data is placed on the replicas, the query will return mostly the same results.

Hostname levenshtein distance

load_balancing = hostname_levenshtein_distance

Just like nearest_hostname, but it compares hostname in a levenshtein distance manner. For example:

example-clickhouse-0-0 ample-clickhouse-0-0
1

example-clickhouse-0-0 example-clickhouse-1-10
2

example-clickhouse-0-0 example-clickhouse-12-0
3

In Order

load_balancing = in_order

Replicas with the same number of errors are accessed in the same order as they are specified in the configuration. This method is appropriate when you know exactly which replica is preferable.

First or Random

load_balancing = first_or_random

This algorithm chooses the first replica in the set or a random replica if the first is unavailable. It’s effective in cross-replication topology setups, but useless in other configurations.

The first_or_random algorithm solves the problem of the in_order algorithm. With in_order, if one replica goes down, the next one gets a double load while the remaining replicas handle the usual amount of traffic. When using the first_or_random algorithm, the load is evenly distributed among replicas that are still available.

It's possible to explicitly define what the first replica is by using the setting load_balancing_first_offset. This gives more control to rebalance query workloads among replicas.

Round Robin

load_balancing = round_robin

This algorithm uses a round-robin policy across replicas with the same number of errors (only the queries with round_robin policy is accounted).

prefer_localhost_replica

Enables/disables preferable using the localhost replica when processing distributed queries.

Possible values:

• 1 — ClickHouse always sends a query to the localhost replica if it exists.
• 0 — ClickHouse uses the balancing strategy specified by the load_balancing setting.

Default value: 1.

:::note Disable this setting if you use max_parallel_replicas without parallel_replicas_custom_key. If parallel_replicas_custom_key is set, disable this setting only if it's used on a cluster with multiple shards containing multiple replicas. If it's used on a cluster with a single shard and multiple replicas, disabling this setting will have negative effects. :::

totals_mode

How to calculate TOTALS when HAVING is present, as well as when max_rows_to_group_by and group_by_overflow_mode = ‘any’ are present. See the section “WITH TOTALS modifier”.

totals_auto_threshold

The threshold for totals_mode = 'auto'. See the section “WITH TOTALS modifier”.

max_parallel_replicas

The maximum number of replicas for each shard when executing a query.

Possible values:

• Positive integer.

Default value: 1.

Additional Info

This options will produce different results depending on the settings used.

:::note This setting will produce incorrect results when joins or subqueries are involved, and all tables don't meet certain requirements. See Distributed Subqueries and max_parallel_replicas for more details. :::

Parallel processing using SAMPLE key

A query may be processed faster if it is executed on several servers in parallel. But the query performance may degrade in the following cases:

• The position of the sampling key in the partitioning key does not allow efficient range scans.
• Adding a sampling key to the table makes filtering by other columns less efficient.
• The sampling key is an expression that is expensive to calculate.
• The cluster latency distribution has a long tail, so that querying more servers increases the query overall latency.

Parallel processing using parallel_replicas_custom_key

This setting is useful for any replicated table.

parallel_replicas_custom_key

An arbitrary integer expression that can be used to split work between replicas for a specific table. The value can be any integer expression. A query may be processed faster if it is executed on several servers in parallel but it depends on the used parallel_replicas_custom_key and parallel_replicas_custom_key_filter_type.

Simple expressions using primary keys are preferred.

If the setting is used on a cluster that consists of a single shard with multiple replicas, those replicas will be converted into virtual shards. Otherwise, it will behave same as for SAMPLE key, it will use multiple replicas of each shard.

parallel_replicas_custom_key_filter_type

How to use parallel_replicas_custom_key expression for splitting work between replicas.

Possible values:

• default — Use the default implementation using modulo operation on the parallel_replicas_custom_key.
• range — Split the entire value space of the expression in the ranges. This type of filtering is useful if values of parallel_replicas_custom_key are uniformly spread across the entire integer space, e.g. hash values.

Default value: default.

allow_experimental_parallel_reading_from_replicas

Enables or disables sending SELECT queries to all replicas of a table (up to max_parallel_replicas). Reading is parallelized and coordinated dynamically. It will work for any kind of MergeTree table.

Possible values:

• 0 - Disabled.
• 1 - Enabled, silently disabled in case of failure.
• 2 - Enabled, throws an exception in case of failure.

Default value: 0.

compile_expressions

Enables or disables compilation of frequently used simple functions and operators to native code with LLVM at runtime.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 1.

min_count_to_compile_expression

Minimum count of executing same expression before it is get compiled.

Default value: 3.

compile_aggregate_expressions

Enables or disables JIT-compilation of aggregate functions to native code. Enabling this setting can improve the performance.

Possible values:

• 0 — Aggregation is done without JIT compilation.
• 1 — Aggregation is done using JIT compilation.

Default value: 1.

See Also

• min_count_to_compile_aggregate_expression

min_count_to_compile_aggregate_expression

The minimum number of identical aggregate expressions to start JIT-compilation. Works only if the compile_aggregate_expressions setting is enabled.

Possible values:

• Positive integer.
• 0 — Identical aggregate expressions are always JIT-compiled.

Default value: 3.

use_query_cache

If turned on, SELECT queries may utilize the query cache. Parameters enable_reads_from_query_cache and enable_writes_to_query_cache control in more detail how the cache is used.

Possible values:

• 0 - Disabled
• 1 - Enabled

Default value: 0.

enable_reads_from_query_cache

If turned on, results of SELECT queries are retrieved from the query cache.

Possible values:

• 0 - Disabled
• 1 - Enabled

Default value: 1.

enable_writes_to_query_cache

If turned on, results of SELECT queries are stored in the query cache.

Possible values:

• 0 - Disabled
• 1 - Enabled

Default value: 1.

query_cache_nondeterministic_function_handling

Controls how the query cache handles SELECT queries with non-deterministic functions like rand() or now().

Possible values:

• 'throw' - Throw an exception and don't cache the query result.
• 'save' - Cache the query result.
• 'ignore' - Don't cache the query result and don't throw an exception.

Default value: throw.

query_cache_system_table_handling

Controls how the query cache handles SELECT queries against system tables, i.e. tables in databases system.* and information_schema.*.

Possible values:

• 'throw' - Throw an exception and don't cache the query result.
• 'save' - Cache the query result.
• 'ignore' - Don't cache the query result and don't throw an exception.

Default value: throw.

query_cache_min_query_runs

Minimum number of times a SELECT query must run before its result is stored in the query cache.

Possible values:

• Positive integer >= 0.

Default value: 0

query_cache_min_query_duration

Minimum duration in milliseconds a query needs to run for its result to be stored in the query cache.

Possible values:

• Positive integer >= 0.

Default value: 0

query_cache_compress_entries

Compress entries in the query cache. Lessens the memory consumption of the query cache at the cost of slower inserts into / reads from it.

Possible values:

• 0 - Disabled
• 1 - Enabled

Default value: 1

query_cache_squash_partial_results

Squash partial result blocks to blocks of size max_block_size. Reduces performance of inserts into the query cache but improves the compressability of cache entries (see query_cache_compress-entries).

Possible values:

• 0 - Disabled
• 1 - Enabled

Default value: 1

query_cache_ttl

After this time in seconds entries in the query cache become stale.

Possible values:

• Positive integer >= 0.

Default value: 60

query_cache_share_between_users

If turned on, the result of SELECT queries cached in the query cache can be read by other users. It is not recommended to enable this setting due to security reasons.

Possible values:

• 0 - Disabled
• 1 - Enabled

Default value: 0.

query_cache_max_size_in_bytes

The maximum amount of memory (in bytes) the current user may allocate in the query cache. 0 means unlimited.

Possible values:

• Positive integer >= 0.

Default value: 0 (no restriction).

query_cache_max_entries

The maximum number of query results the current user may store in the query cache. 0 means unlimited.

Possible values:

• Positive integer >= 0.

Default value: 0 (no restriction).

insert_quorum

:::note This setting is not applicable to SharedMergeTree, see SharedMergeTree consistency for more information. :::

Enables the quorum writes.

• If insert_quorum < 2, the quorum writes are disabled.
• If insert_quorum >= 2, the quorum writes are enabled.
• If insert_quorum = 'auto', use majority number (number_of_replicas / 2 + 1) as quorum number.

Default value: 0 - disabled.

Quorum writes

INSERT succeeds only when ClickHouse manages to correctly write data to the insert_quorum of replicas during the insert_quorum_timeout. If for any reason the number of replicas with successful writes does not reach the insert_quorum, the write is considered failed and ClickHouse will delete the inserted block from all the replicas where data has already been written.

When insert_quorum_parallel is disabled, all replicas in the quorum are consistent, i.e. they contain data from all previous INSERT queries (the INSERT sequence is linearized). When reading data written using insert_quorum and insert_quorum_parallel is disabled, you can turn on sequential consistency for SELECT queries using select_sequential_consistency.

ClickHouse generates an exception:

• If the number of available replicas at the time of the query is less than the insert_quorum.
• When insert_quorum_parallel is disabled and an attempt to write data is made when the previous block has not yet been inserted in insert_quorum of replicas. This situation may occur if the user tries to perform another INSERT query to the same table before the previous one with insert_quorum is completed.

See also:

• insert_quorum_timeout
• insert_quorum_parallel
• select_sequential_consistency

insert_quorum_timeout

Write to a quorum timeout in milliseconds. If the timeout has passed and no write has taken place yet, ClickHouse will generate an exception and the client must repeat the query to write the same block to the same or any other replica.

Default value: 600 000 milliseconds (ten minutes).

See also:

• insert_quorum
• insert_quorum_parallel
• select_sequential_consistency

insert_quorum_parallel

:::note This setting is not applicable to SharedMergeTree, see SharedMergeTree consistency for more information. :::

Enables or disables parallelism for quorum INSERT queries. If enabled, additional INSERT queries can be sent while previous queries have not yet finished. If disabled, additional writes to the same table will be rejected.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 1.

See also:

• insert_quorum
• insert_quorum_timeout
• select_sequential_consistency

select_sequential_consistency

:::note This setting differ in behavior between SharedMergeTree and ReplicatedMergeTree, see SharedMergeTree consistency for more information about the behavior of select_sequential_consistency in SharedMergeTree. :::

Enables or disables sequential consistency for SELECT queries. Requires insert_quorum_parallel to be disabled (enabled by default).

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 0.

Usage

When sequential consistency is enabled, ClickHouse allows the client to execute the SELECT query only for those replicas that contain data from all previous INSERT queries executed with insert_quorum. If the client refers to a partial replica, ClickHouse will generate an exception. The SELECT query will not include data that has not yet been written to the quorum of replicas.

When insert_quorum_parallel is enabled (the default), then select_sequential_consistency does not work. This is because parallel INSERT queries can be written to different sets of quorum replicas so there is no guarantee a single replica will have received all writes.

See also:

• insert_quorum
• insert_quorum_timeout
• insert_quorum_parallel

insert_deduplicate

Enables or disables block deduplication of INSERT (for Replicated* tables).

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 1.

By default, blocks inserted into replicated tables by the INSERT statement are deduplicated (see Data Replication). For the replicated tables by default the only 100 of the most recent blocks for each partition are deduplicated (see replicated_deduplication_window, replicated_deduplication_window_seconds). For not replicated tables see non_replicated_deduplication_window.

Asynchronous Insert settings

async_insert

Enables or disables asynchronous inserts. Note that deduplication is disabled by default, see async_insert_deduplicate.

If enabled, the data is combined into batches before the insertion into tables, so it is possible to do small and frequent insertions into ClickHouse (up to 15000 queries per second) without buffer tables.

The data is inserted either after the async_insert_max_data_size is exceeded or after async_insert_busy_timeout_ms milliseconds since the first INSERT query. If the async_insert_stale_timeout_ms is set to a non-zero value, the data is inserted after async_insert_stale_timeout_ms milliseconds since the last query. Also the buffer will be flushed to disk if at least async_insert_max_query_number async insert queries per block were received. This last setting takes effect only if async_insert_deduplicate is enabled.

If wait_for_async_insert is enabled, every client will wait for the data to be processed and flushed to the table. Otherwise, the query would be processed almost instantly, even if the data is not inserted.

Possible values:

• 0 — Insertions are made synchronously, one after another.
• 1 — Multiple asynchronous insertions enabled.

Default value: 0.

async_insert_threads

The maximum number of threads for background data parsing and insertion.

Possible values:

• Positive integer.
• 0 — Asynchronous insertions are disabled.

Default value: 16.

wait_for_async_insert

Enables or disables waiting for processing of asynchronous insertion. If enabled, server will return OK only after the data is inserted. Otherwise, it will return OK as soon it has received the data, but it might still fail to parse or insert it later (You can check in system.asynchronous_insert_log)

If you want to use asynchronous inserts, we need to also enable async_insert.

Possible values:

• 0 — Server returns OK even if the data is not yet inserted.
• 1 — Server returns OK only after the data is inserted.

Default value: 1.

wait_for_async_insert_timeout

The timeout in seconds for waiting for processing of asynchronous insertion.

Possible values:

• Positive integer.
• 0 — Disabled.

Default value: lock_acquire_timeout.

async_insert_max_data_size

The maximum size of the unparsed data in bytes collected per query before being inserted.

Possible values:

• Positive integer.
• 0 — Asynchronous insertions are disabled.

Default value: 1000000.

async_insert_max_query_number

The maximum number of insert queries per block before being inserted. This setting takes effect only if async_insert_deduplicate is enabled.

Possible values:

• Positive integer.
• 0 — Asynchronous insertions are disabled.

Default value: 450.

async_insert_busy_timeout_max_ms

The maximum timeout in milliseconds since the first INSERT query before inserting collected data.

Possible values:

• Positive integer.
• 0 — Timeout disabled.

Default value: 200.

Cloud default value: 1000.

async_insert_poll_timeout_ms

Timeout in milliseconds for polling data from asynchronous insert queue.

Possible values:

• Positive integer.

Default value: 10.

async_insert_use_adaptive_busy_timeout

Use adaptive asynchronous insert timeout.

Possible values:

• 0 - Disabled.
• 1 - Enabled.

Default value: 0.

async_insert_busy_timeout_min_ms

If adaptive asynchronous insert timeout is allowed through async_insert_use_adaptive_busy_timeout, the setting specifies the minimum value of the asynchronous insert timeout in milliseconds. It also serves as the initial value, which may be increased later by the adaptive algorithm, up to the async_insert_busy_timeout_ms.

Possible values:

• Positive integer.

Default value: 50.

async_insert_busy_timeout_ms

Alias for async_insert_busy_timeout_max_ms.

async_insert_busy_timeout_increase_rate

If adaptive asynchronous insert timeout is allowed through async_insert_use_adaptive_busy_timeout, the setting specifies the exponential growth rate at which the adaptive asynchronous insert timeout increases.

Possible values:

• A positive floating-point number.

Default value: 0.2.

async_insert_busy_timeout_decrease_rate

If adaptive asynchronous insert timeout is allowed through async_insert_use_adaptive_busy_timeout, the setting specifies the exponential growth rate at which the adaptive asynchronous insert timeout decreases.

Possible values:

• A positive floating-point number.

Default value: 0.2.

async_insert_stale_timeout_ms

The maximum timeout in milliseconds since the last INSERT query before dumping collected data. If enabled, the settings prolongs the async_insert_busy_timeout_ms with every INSERT query as long as async_insert_max_data_size is not exceeded.

Possible values:

• Positive integer.
• 0 — Timeout disabled.

Default value: 0.

async_insert_deduplicate

Enables or disables insert deduplication of ASYNC INSERT (for Replicated* tables).

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 0.

By default, async inserts are inserted into replicated tables by the INSERT statement enabling async_insert are deduplicated (see Data Replication). For the replicated tables, by default, only 10000 of the most recent inserts for each partition are deduplicated (see replicated_deduplication_window_for_async_inserts, replicated_deduplication_window_seconds_for_async_inserts). We recommend enabling the async_block_ids_cache to increase the efficiency of deduplication. This function does not work for non-replicated tables.

deduplicate_blocks_in_dependent_materialized_views

Enables or disables the deduplication check for materialized views that receive data from Replicated* tables.

Possible values:

  0 — Disabled.
  1 — Enabled.

Default value: 0.

Usage

By default, deduplication is not performed for materialized views but is done upstream, in the source table. If an INSERTed block is skipped due to deduplication in the source table, there will be no insertion into attached materialized views. This behaviour exists to enable the insertion of highly aggregated data into materialized views, for cases where inserted blocks are the same after materialized view aggregation but derived from different INSERTs into the source table. At the same time, this behaviour “breaks” INSERT idempotency. If an INSERT into the main table was successful and INSERT into a materialized view failed (e.g. because of communication failure with ClickHouse Keeper) a client will get an error and can retry the operation. However, the materialized view won’t receive the second insert because it will be discarded by deduplication in the main (source) table. The setting deduplicate_blocks_in_dependent_materialized_views allows for changing this behaviour. On retry, a materialized view will receive the repeat insert and will perform a deduplication check by itself, ignoring check result for the source table, and will insert rows lost because of the first failure.

insert_deduplication_token

The setting allows a user to provide own deduplication semantic in MergeTree/ReplicatedMergeTree For example, by providing a unique value for the setting in each INSERT statement, user can avoid the same inserted data being deduplicated.

Possible values:

• Any string

Default value: empty string (disabled)

insert_deduplication_token is used for deduplication only when not empty.

For the replicated tables by default the only 100 of the most recent inserts for each partition are deduplicated (see replicated_deduplication_window, replicated_deduplication_window_seconds). For not replicated tables see non_replicated_deduplication_window.

:::note insert_deduplication_token works on a partition level (the same as insert_deduplication checksum). Multiple partitions can have the same insert_deduplication_token. :::

Example:

CREATE TABLE test_table
( A Int64 )
ENGINE = MergeTree
ORDER BY A
SETTINGS non_replicated_deduplication_window = 100;

INSERT INTO test_table SETTINGS insert_deduplication_token = 'test' VALUES (1);

-- the next insert won't be deduplicated because insert_deduplication_token is different
INSERT INTO test_table SETTINGS insert_deduplication_token = 'test1' VALUES (1);

-- the next insert will be deduplicated because insert_deduplication_token
-- is the same as one of the previous
INSERT INTO test_table SETTINGS insert_deduplication_token = 'test' VALUES (2);

SELECT * FROM test_table

┌─A─┐
│ 1 │
└───┘
┌─A─┐
│ 1 │
└───┘

update_insert_deduplication_token_in_dependent_materialized_views

Allows to update insert_deduplication_token with view identifier during insert in dependent materialized views, if setting deduplicate_blocks_in_dependent_materialized_views is enabled and insert_deduplication_token is set.

Possible values:

  0 — Disabled.
  1 — Enabled.

Default value: 0.

Usage:

If setting deduplicate_blocks_in_dependent_materialized_views is enabled, insert_deduplication_token is passed to dependent materialized views. But in complex INSERT flows it is possible that we want to avoid deduplication for dependent materialized views.

Example:

landing -┬--> mv_1_1 ---> ds_1_1 ---> mv_2_1 --┬-> ds_2_1 ---> mv_3_1 ---> ds_3_1
         |                                     |
         └--> mv_1_2 ---> ds_1_2 ---> mv_2_2 --┘

In this example we want to avoid deduplication for two different blocks generated from mv_2_1 and mv_2_2 that will be inserted into ds_2_1. Without update_insert_deduplication_token_in_dependent_materialized_views setting enabled, those two different blocks will be deduplicated, because different blocks from mv_2_1 and mv_2_2 will have the same insert_deduplication_token.

If setting update_insert_deduplication_token_in_dependent_materialized_views is enabled, during each insert into dependent materialized views insert_deduplication_token is updated with table identifier, so block from mv_2_1 and block from mv_2_2 will have different insert_deduplication_token and will not be deduplicated.

insert_keeper_max_retries

The setting sets the maximum number of retries for ClickHouse Keeper (or ZooKeeper) requests during insert into replicated MergeTree. Only Keeper requests which failed due to network error, Keeper session timeout, or request timeout are considered for retries.

Possible values:

• Positive integer.
• 0 — Retries are disabled

Default value: 20

Cloud default value: 20.

Keeper request retries are done after some timeout. The timeout is controlled by the following settings: insert_keeper_retry_initial_backoff_ms, insert_keeper_retry_max_backoff_ms. The first retry is done after insert_keeper_retry_initial_backoff_ms timeout. The consequent timeouts will be calculated as follows:

timeout = min(insert_keeper_retry_max_backoff_ms, latest_timeout * 2)

For example, if insert_keeper_retry_initial_backoff_ms=100, insert_keeper_retry_max_backoff_ms=10000 and insert_keeper_max_retries=8 then timeouts will be 100, 200, 400, 800, 1600, 3200, 6400, 10000.

Apart from fault tolerance, the retries aim to provide a better user experience - they allow to avoid returning an error during INSERT execution if Keeper is restarted, for example, due to an upgrade.

insert_keeper_retry_initial_backoff_ms

Initial timeout(in milliseconds) to retry a failed Keeper request during INSERT query execution

Possible values:

• Positive integer.
• 0 — No timeout

Default value: 100

insert_keeper_retry_max_backoff_ms

Maximum timeout (in milliseconds) to retry a failed Keeper request during INSERT query execution

Possible values:

• Positive integer.
• 0 — Maximum timeout is not limited

Default value: 10000

max_network_bytes

Limits the data volume (in bytes) that is received or transmitted over the network when executing a query. This setting applies to every individual query.

Possible values:

• Positive integer.
• 0 — Data volume control is disabled.

Default value: 0.

max_network_bandwidth

Limits the speed of the data exchange over the network in bytes per second. This setting applies to every query.

Possible values:

• Positive integer.
• 0 — Bandwidth control is disabled.

Default value: 0.

max_network_bandwidth_for_user

Limits the speed of the data exchange over the network in bytes per second. This setting applies to all concurrently running queries performed by a single user.

Possible values:

• Positive integer.
• 0 — Control of the data speed is disabled.

Default value: 0.

max_network_bandwidth_for_all_users

Limits the speed that data is exchanged at over the network in bytes per second. This setting applies to all concurrently running queries on the server.

Possible values:

• Positive integer.
• 0 — Control of the data speed is disabled.

Default value: 0.

count_distinct_implementation

Specifies which of the uniq* functions should be used to perform the COUNT(DISTINCT …) construction.

Possible values:

• uniq
• uniqCombined
• uniqCombined64
• uniqHLL12
• uniqExact

Default value: uniqExact.

skip_unavailable_shards

Enables or disables silently skipping of unavailable shards.

Shard is considered unavailable if all its replicas are unavailable. A replica is unavailable in the following cases:

• ClickHouse can’t connect to replica for any reason.

  When connecting to a replica, ClickHouse performs several attempts. If all these attempts fail, the replica is considered unavailable.

• Replica can’t be resolved through DNS.

  If replica’s hostname can’t be resolved through DNS, it can indicate the following situations:

  • Replica’s host has no DNS record. It can occur in systems with dynamic DNS, for example, Kubernetes, where nodes can be unresolvable during downtime, and this is not an error.

  • Configuration error. ClickHouse configuration file contains a wrong hostname.

Possible values:

• 1 — skipping enabled.

  If a shard is unavailable, ClickHouse returns a result based on partial data and does not report node availability issues.

• 0 — skipping disabled.

  If a shard is unavailable, ClickHouse throws an exception.

Default value: 0.

distributed_group_by_no_merge

Do not merge aggregation states from different servers for distributed query processing, you can use this in case it is for certain that there are different keys on different shards

Possible values:

• 0 — Disabled (final query processing is done on the initiator node).
• 1 - Do not merge aggregation states from different servers for distributed query processing (query completely processed on the shard, initiator only proxy the data), can be used in case it is for certain that there are different keys on different shards.
• 2 - Same as 1 but applies ORDER BY and LIMIT (it is not possible when the query processed completely on the remote node, like for distributed_group_by_no_merge=1) on the initiator (can be used for queries with ORDER BY and/or LIMIT).

Default value: 0

Example

SELECT *
FROM remote('127.0.0.{2,3}', system.one)
GROUP BY dummy
LIMIT 1
SETTINGS distributed_group_by_no_merge = 1
FORMAT PrettyCompactMonoBlock

┌─dummy─┐
│     0 │
│     0 │
└───────┘

SELECT *
FROM remote('127.0.0.{2,3}', system.one)
GROUP BY dummy
LIMIT 1
SETTINGS distributed_group_by_no_merge = 2
FORMAT PrettyCompactMonoBlock

┌─dummy─┐
│     0 │
└───────┘

distributed_push_down_limit

Enables or disables LIMIT applying on each shard separately.

This will allow to avoid:

• Sending extra rows over network;
• Processing rows behind the limit on the initiator.

Starting from 21.9 version you cannot get inaccurate results anymore, since distributed_push_down_limit changes query execution only if at least one of the conditions met:

• distributed_group_by_no_merge > 0.
• Query does not have GROUP BY/DISTINCT/LIMIT BY, but it has ORDER BY/LIMIT.
• Query has GROUP BY/DISTINCT/LIMIT BY with ORDER BY/LIMIT and:
  • optimize_skip_unused_shards is enabled.
  • optimize_distributed_group_by_sharding_key is enabled.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 1.

See also:

• distributed_group_by_no_merge
• optimize_skip_unused_shards
• optimize_distributed_group_by_sharding_key

optimize_skip_unused_shards_limit

Limit for number of sharding key values, turns off optimize_skip_unused_shards if the limit is reached.

Too many values may require significant amount for processing, while the benefit is doubtful, since if you have huge number of values in IN (...), then most likely the query will be sent to all shards anyway.

Default value: 1000

optimize_skip_unused_shards

Enables or disables skipping of unused shards for SELECT queries that have sharding key condition in WHERE/PREWHERE (assuming that the data is distributed by sharding key, otherwise a query yields incorrect result).

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 0

optimize_skip_unused_shards_rewrite_in

Rewrite IN in query for remote shards to exclude values that does not belong to the shard (requires optimize_skip_unused_shards).

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 1 (since it requires optimize_skip_unused_shards anyway, which 0 by default)

allow_nondeterministic_optimize_skip_unused_shards

Allow nondeterministic (like rand or dictGet, since later has some caveats with updates) functions in sharding key.

Possible values:

• 0 — Disallowed.
• 1 — Allowed.

Default value: 0

optimize_skip_unused_shards_nesting

Controls optimize_skip_unused_shards (hence still requires optimize_skip_unused_shards) depends on the nesting level of the distributed query (case when you have Distributed table that look into another Distributed table).

Possible values:

• 0 — Disabled, optimize_skip_unused_shards works always.
• 1 — Enables optimize_skip_unused_shards only for the first level.
• 2 — Enables optimize_skip_unused_shards up to the second level.

Default value: 0

force_optimize_skip_unused_shards

Enables or disables query execution if optimize_skip_unused_shards is enabled and skipping of unused shards is not possible. If the skipping is not possible and the setting is enabled, an exception will be thrown.

Possible values:

• 0 — Disabled. ClickHouse does not throw an exception.
• 1 — Enabled. Query execution is disabled only if the table has a sharding key.
• 2 — Enabled. Query execution is disabled regardless of whether a sharding key is defined for the table.

Default value: 0

force_optimize_skip_unused_shards_nesting

Controls force_optimize_skip_unused_shards (hence still requires force_optimize_skip_unused_shards) depends on the nesting level of the distributed query (case when you have Distributed table that look into another Distributed table).

Possible values:

• 0 - Disabled, force_optimize_skip_unused_shards works always.
• 1 — Enables force_optimize_skip_unused_shards only for the first level.
• 2 — Enables force_optimize_skip_unused_shards up to the second level.

Default value: 0

optimize_distributed_group_by_sharding_key

Optimize GROUP BY sharding_key queries, by avoiding costly aggregation on the initiator server (which will reduce memory usage for the query on the initiator server).

The following types of queries are supported (and all combinations of them):

• SELECT DISTINCT [..., ]sharding_key[, ...] FROM dist
• SELECT ... FROM dist GROUP BY sharding_key[, ...]
• SELECT ... FROM dist GROUP BY sharding_key[, ...] ORDER BY x
• SELECT ... FROM dist GROUP BY sharding_key[, ...] LIMIT 1
• SELECT ... FROM dist GROUP BY sharding_key[, ...] LIMIT 1 BY x

The following types of queries are not supported (support for some of them may be added later):

• SELECT ... GROUP BY sharding_key[, ...] WITH TOTALS
• SELECT ... GROUP BY sharding_key[, ...] WITH ROLLUP
• SELECT ... GROUP BY sharding_key[, ...] WITH CUBE
• SELECT ... GROUP BY sharding_key[, ...] SETTINGS extremes=1

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 0

See also:

• distributed_group_by_no_merge
• distributed_push_down_limit
• optimize_skip_unused_shards

:::note Right now it requires optimize_skip_unused_shards (the reason behind this is that one day it may be enabled by default, and it will work correctly only if data was inserted via Distributed table, i.e. data is distributed according to sharding_key). :::

optimize_throw_if_noop

Enables or disables throwing an exception if an OPTIMIZE query didn’t perform a merge.

By default, OPTIMIZE returns successfully even if it didn’t do anything. This setting lets you differentiate these situations and get the reason in an exception message.

Possible values:

• 1 — Throwing an exception is enabled.
• 0 — Throwing an exception is disabled.

Default value: 0.

optimize_skip_merged_partitions

Enables or disables optimization for OPTIMIZE TABLE ... FINAL query if there is only one part with level > 0 and it doesn't have expired TTL.

• OPTIMIZE TABLE ... FINAL SETTINGS optimize_skip_merged_partitions=1

By default, OPTIMIZE TABLE ... FINAL query rewrites the one part even if there is only a single part.

Possible values:

• 1 - Enable optimization.
• 0 - Disable optimization.

Default value: 0.

optimize_functions_to_subcolumns

Enables or disables optimization by transforming some functions to reading subcolumns. This reduces the amount of data to read.

These functions can be transformed:

• length to read the size0 subcolumn.
• empty to read the size0 subcolumn.
• notEmpty to read the size0 subcolumn.
• isNull to read the null subcolumn.
• isNotNull to read the null subcolumn.
• count to read the null subcolumn.
• mapKeys to read the keys subcolumn.
• mapValues to read the values subcolumn.

Possible values:

• 0 — Optimization disabled.
• 1 — Optimization enabled.

Default value: 0.

optimize_trivial_count_query

Enables or disables the optimization to trivial query SELECT count() FROM table using metadata from MergeTree. If you need to use row-level security, disable this setting.

Possible values:

• 0 — Optimization disabled.
• 1 — Optimization enabled.

Default value: 1.

See also:

• optimize_functions_to_subcolumns

optimize_trivial_approximate_count_query

Use an approximate value for trivial count optimization of storages that support such estimation, for example, EmbeddedRocksDB.

Possible values:

• 0 — Optimization disabled.
• 1 — Optimization enabled.

Default value: 0.

optimize_count_from_files

Enables or disables the optimization of counting number of rows from files in different input formats. It applies to table functions/engines file/s3/url/hdfs/azureBlobStorage.

Possible values:

• 0 — Optimization disabled.
• 1 — Optimization enabled.

Default value: 1.

use_cache_for_count_from_files

Enables caching of rows number during count from files in table functions file/s3/url/hdfs/azureBlobStorage.

Enabled by default.

distributed_replica_error_half_life

• Type: seconds
• Default value: 60 seconds

Controls how fast errors in distributed tables are zeroed. If a replica is unavailable for some time, accumulates 5 errors, and distributed_replica_error_half_life is set to 1 second, then the replica is considered normal 3 seconds after the last error.

See also:

• load_balancing
• Table engine Distributed
• distributed_replica_error_cap
• distributed_replica_max_ignored_errors

distributed_replica_error_cap

• Type: unsigned int
• Default value: 1000

The error count of each replica is capped at this value, preventing a single replica from accumulating too many errors.

See also:

• load_balancing
• Table engine Distributed
• distributed_replica_error_half_life
• distributed_replica_max_ignored_errors

distributed_replica_max_ignored_errors

• Type: unsigned int
• Default value: 0

The number of errors that will be ignored while choosing replicas (according to load_balancing algorithm).

See also:

• load_balancing
• Table engine Distributed
• distributed_replica_error_cap
• distributed_replica_error_half_life

distributed_background_insert_sleep_time_ms

Base interval for the Distributed table engine to send data. The actual interval grows exponentially in the event of errors.

Possible values:

• A positive integer number of milliseconds.

Default value: 100 milliseconds.

distributed_background_insert_max_sleep_time_ms

Maximum interval for the Distributed table engine to send data. Limits exponential growth of the interval set in the distributed_background_insert_sleep_time_ms setting.

Possible values:

• A positive integer number of milliseconds.

Default value: 30000 milliseconds (30 seconds).

distributed_background_insert_batch

Enables/disables inserted data sending in batches.

When batch sending is enabled, the Distributed table engine tries to send multiple files of inserted data in one operation instead of sending them separately. Batch sending improves cluster performance by better-utilizing server and network resources.

Possible values:

• 1 — Enabled.
• 0 — Disabled.

Default value: 0.

distributed_background_insert_split_batch_on_failure

Enables/disables splitting batches on failures.

Sometimes sending particular batch to the remote shard may fail, because of some complex pipeline after (i.e. MATERIALIZED VIEW with GROUP BY) due to Memory limit exceeded or similar errors. In this case, retrying will not help (and this will stuck distributed sends for the table) but sending files from that batch one by one may succeed INSERT.

So installing this setting to 1 will disable batching for such batches (i.e. temporary disables distributed_background_insert_batch for failed batches).

Possible values:

• 1 — Enabled.
• 0 — Disabled.

Default value: 0.

:::note This setting also affects broken batches (that may appears because of abnormal server (machine) termination and no fsync_after_insert/fsync_directories for Distributed table engine). :::

:::note You should not rely on automatic batch splitting, since this may hurt performance. :::

os_thread_priority

Sets the priority (nice) for threads that execute queries. The OS scheduler considers this priority when choosing the next thread to run on each available CPU core.

:::note To use this setting, you need to set the CAP_SYS_NICE capability. The clickhouse-server package sets it up during installation. Some virtual environments do not allow you to set the CAP_SYS_NICE capability. In this case, clickhouse-server shows a message about it at the start. :::

Possible values:

• You can set values in the range [-20, 19].

Lower values mean higher priority. Threads with low nice priority values are executed more frequently than threads with high values. High values are preferable for long-running non-interactive queries because it allows them to quickly give up resources in favour of short interactive queries when they arrive.

Default value: 0.

query_profiler_real_time_period_ns

Sets the period for a real clock timer of the query profiler. Real clock timer counts wall-clock time.

Possible values:

• Positive integer number, in nanoseconds.

  Recommended values:

        - 10000000 (100 times a second) nanoseconds and less for single queries.
        - 1000000000 (once a second) for cluster-wide profiling.
  

• 0 for turning off the timer.

Type: UInt64.

Default value: 1000000000 nanoseconds (once a second).

Temporarily disabled in ClickHouse Cloud.

See also:

• System table trace_log

query_profiler_cpu_time_period_ns

Sets the period for a CPU clock timer of the query profiler. This timer counts only CPU time.

Possible values:

• A positive integer number of nanoseconds.

  Recommended values:

        - 10000000 (100 times a second) nanoseconds and more for single queries.
        - 1000000000 (once a second) for cluster-wide profiling.
  

• 0 for turning off the timer.

Type: UInt64.

Default value: 1000000000 nanoseconds.

Temporarily disabled in ClickHouse Cloud.

See also:

• System table trace_log

memory_profiler_step

Sets the step of memory profiler. Whenever query memory usage becomes larger than every next step in number of bytes the memory profiler will collect the allocating stacktrace and will write it into trace_log.

Possible values:

• A positive integer number of bytes.

• 0 for turning off the memory profiler.

Default value: 4,194,304 bytes (4 MiB).

memory_profiler_sample_probability

Sets the probability of collecting stacktraces at random allocations and deallocations and writing them into trace_log.

Possible values:

• A positive floating-point number in the range [0..1].

• 0.0 for turning off the memory sampling.

Default value: 0.0.

trace_profile_events

Enables or disables collecting stacktraces on each update of profile events along with the name of profile event and the value of increment and sending them into trace_log.

Possible values:

• 1 — Tracing of profile events enabled.
• 0 — Tracing of profile events disabled.

Default value: 0.

allow_introspection_functions

Enables or disables introspection functions for query profiling.

Possible values:

• 1 — Introspection functions enabled.
• 0 — Introspection functions disabled.

Default value: 0.

See Also

• Sampling Query Profiler
• System table trace_log

input_format_parallel_parsing

Enables or disables order-preserving parallel parsing of data formats. Supported only for TSV, TSKV, CSV and JSONEachRow formats.

Possible values:

• 1 — Enabled.
• 0 — Disabled.

Default value: 1.

output_format_parallel_formatting

Enables or disables parallel formatting of data formats. Supported only for TSV, TSKV, CSV and JSONEachRow formats.

Possible values:

• 1 — Enabled.
• 0 — Disabled.

Default value: 1.

min_chunk_bytes_for_parallel_parsing

• Type: unsigned int
• Default value: 1 MiB

The minimum chunk size in bytes, which each thread will parse in parallel.

merge_selecting_sleep_ms

Sleep time for merge selecting when no part is selected. A lower setting triggers selecting tasks in background_schedule_pool frequently, which results in a large number of requests to ClickHouse Keeper in large-scale clusters.

Possible values:

• Any positive integer.

Default value: 5000.

parallel_distributed_insert_select

Enables parallel distributed INSERT ... SELECT query.

If we execute INSERT INTO distributed_table_a SELECT ... FROM distributed_table_b queries and both tables use the same cluster, and both tables are either replicated or non-replicated, then this query is processed locally on every shard.

Possible values:

• 0 — Disabled.
• 1 — SELECT will be executed on each shard from the underlying table of the distributed engine.
• 2 — SELECT and INSERT will be executed on each shard from/to the underlying table of the distributed engine.

Default value: 0.

distributed_insert_skip_read_only_replicas

Enables skipping read-only replicas for INSERT queries into Distributed.

Possible values:

• 0 — INSERT was as usual, if it will go to read-only replica it will fail
• 1 — Initiator will skip read-only replicas before sending data to shards.

Default value: 0

distributed_foreground_insert

Enables or disables synchronous data insertion into a Distributed table.

By default, when inserting data into a Distributed table, the ClickHouse server sends data to cluster nodes in background mode. When distributed_foreground_insert=1, the data is processed synchronously, and the INSERT operation succeeds only after all the data is saved on all shards (at least one replica for each shard if internal_replication is true).

Possible values:

• 0 — Data is inserted in background mode.
• 1 — Data is inserted in synchronous mode.

Default value: 0.

Cloud default value: 1.

See Also

• Distributed Table Engine
• Managing Distributed Tables

insert_distributed_sync

Alias for distributed_foreground_insert.

insert_shard_id

If not 0, specifies the shard of Distributed table into which the data will be inserted synchronously.

If insert_shard_id value is incorrect, the server will throw an exception.

To get the number of shards on requested_cluster, you can check server config or use this query:

SELECT uniq(shard_num) FROM system.clusters WHERE cluster = 'requested_cluster';

Possible values:

• 0 — Disabled.
• Any number from 1 to shards_num of corresponding Distributed table.

Default value: 0.

Example

Query:

CREATE TABLE x AS system.numbers ENGINE = MergeTree ORDER BY number;
CREATE TABLE x_dist AS x ENGINE = Distributed('test_cluster_two_shards_localhost', currentDatabase(), x);
INSERT INTO x_dist SELECT * FROM numbers(5) SETTINGS insert_shard_id = 1;
SELECT * FROM x_dist ORDER BY number ASC;

Result:

┌─number─┐
│      0 │
│      0 │
│      1 │
│      1 │
│      2 │
│      2 │
│      3 │
│      3 │
│      4 │
│      4 │
└────────┘

use_compact_format_in_distributed_parts_names

Uses compact format for storing blocks for background (distributed_foreground_insert) INSERT into tables with Distributed engine.

Possible values:

• 0 — Uses user[:password]@host:port#default_database directory format.
• 1 — Uses [shard{shard_index}[_replica{replica_index}]] directory format.

Default value: 1.

:::note

• with use_compact_format_in_distributed_parts_names=0 changes from cluster definition will not be applied for background INSERT.
• with use_compact_format_in_distributed_parts_names=1 changing the order of the nodes in the cluster definition, will change the shard_index/replica_index so be aware. :::

background_buffer_flush_schedule_pool_size

That setting was moved to the server configuration parameters.

background_move_pool_size

That setting was moved to the server configuration parameters.

background_schedule_pool_size

That setting was moved to the server configuration parameters.

background_fetches_pool_size

That setting was moved to the server configuration parameters.

always_fetch_merged_part

Prohibits data parts merging in Replicated*MergeTree-engine tables.

When merging is prohibited, the replica never merges parts and always downloads merged parts from other replicas. If there is no required data yet, the replica waits for it. CPU and disk load on the replica server decreases, but the network load on the cluster increases. This setting can be useful on servers with relatively weak CPUs or slow disks, such as servers for backups storage.

Possible values:

• 0 — Replicated*MergeTree-engine tables merge data parts at the replica.
• 1 — Replicated*MergeTree-engine tables do not merge data parts at the replica. The tables download merged data parts from other replicas.

Default value: 0.

See Also

• Data Replication

background_distributed_schedule_pool_size

That setting was moved to the server configuration parameters.

background_message_broker_schedule_pool_size

That setting was moved to the server configuration parameters.

validate_polygons

Enables or disables throwing an exception in the pointInPolygon function, if the polygon is self-intersecting or self-tangent.

Possible values:

• 0 — Throwing an exception is disabled. pointInPolygon accepts invalid polygons and returns possibly incorrect results for them.
• 1 — Throwing an exception is enabled.

Default value: 1.

transform_null_in

Enables equality of NULL values for IN operator.

By default, NULL values can’t be compared because NULL means undefined value. Thus, comparison expr = NULL must always return false. With this setting NULL = NULL returns true for IN operator.

Possible values:

• 0 — Comparison of NULL values in IN operator returns false.
• 1 — Comparison of NULL values in IN operator returns true.

Default value: 0.

Example

Consider the null_in table:

┌──idx─┬─────i─┐
│    1 │     1 │
│    2 │  NULL │
│    3 │     3 │
└──────┴───────┘

Query:

SELECT idx, i FROM null_in WHERE i IN (1, NULL) SETTINGS transform_null_in = 0;

Result:

┌──idx─┬────i─┐
│    1 │    1 │
└──────┴──────┘

Query:

SELECT idx, i FROM null_in WHERE i IN (1, NULL) SETTINGS transform_null_in = 1;

Result:

┌──idx─┬─────i─┐
│    1 │     1 │
│    2 │  NULL │
└──────┴───────┘

See Also

• NULL Processing in IN Operators

low_cardinality_max_dictionary_size

Sets a maximum size in rows of a shared global dictionary for the LowCardinality data type that can be written to a storage file system. This setting prevents issues with RAM in case of unlimited dictionary growth. All the data that can’t be encoded due to maximum dictionary size limitation ClickHouse writes in an ordinary method.

Possible values:

• Any positive integer.

Default value: 8192.

low_cardinality_use_single_dictionary_for_part

Turns on or turns off using of single dictionary for the data part.

By default, the ClickHouse server monitors the size of dictionaries and if a dictionary overflows then the server starts to write the next one. To prohibit creating several dictionaries set low_cardinality_use_single_dictionary_for_part = 1.

Possible values:

• 1 — Creating several dictionaries for the data part is prohibited.
• 0 — Creating several dictionaries for the data part is not prohibited.

Default value: 0.

low_cardinality_allow_in_native_format

Allows or restricts using the LowCardinality data type with the Native format.

If usage of LowCardinality is restricted, ClickHouse server converts LowCardinality-columns to ordinary ones for SELECT queries, and convert ordinary columns to LowCardinality-columns for INSERT queries.

This setting is required mainly for third-party clients which do not support LowCardinality data type.

Possible values:

• 1 — Usage of LowCardinality is not restricted.
• 0 — Usage of LowCardinality is restricted.

Default value: 1.

allow_suspicious_low_cardinality_types

Allows or restricts using LowCardinality with data types with fixed size of 8 bytes or less: numeric data types and FixedString(8_bytes_or_less).

For small fixed values using of LowCardinality is usually inefficient, because ClickHouse stores a numeric index for each row. As a result:

• Disk space usage can rise.
• RAM consumption can be higher, depending on a dictionary size.
• Some functions can work slower due to extra coding/encoding operations.

Merge times in MergeTree-engine tables can grow due to all the reasons described above.

Possible values:

• 1 — Usage of LowCardinality is not restricted.
• 0 — Usage of LowCardinality is restricted.

Default value: 0.

min_insert_block_size_rows_for_materialized_views

Sets the minimum number of rows in the block which can be inserted into a table by an INSERT query. Smaller-sized blocks are squashed into bigger ones. This setting is applied only for blocks inserted into materialized view. By adjusting this setting, you control blocks squashing while pushing to materialized view and avoid excessive memory usage.

Possible values:

• Any positive integer.
• 0 — Squashing disabled.

Default value: 1048576.

See Also

• min_insert_block_size_rows

min_insert_block_size_bytes_for_materialized_views

Sets the minimum number of bytes in the block which can be inserted into a table by an INSERT query. Smaller-sized blocks are squashed into bigger ones. This setting is applied only for blocks inserted into materialized view. By adjusting this setting, you control blocks squashing while pushing to materialized view and avoid excessive memory usage.

Possible values:

• Any positive integer.
• 0 — Squashing disabled.

Default value: 268435456.

See also

• min_insert_block_size_bytes

optimize_read_in_order

Enables ORDER BY optimization in SELECT queries for reading data from MergeTree tables.

Possible values:

• 0 — ORDER BY optimization is disabled.
• 1 — ORDER BY optimization is enabled.

Default value: 1.

See Also

• ORDER BY Clause

optimize_aggregation_in_order

Enables GROUP BY optimization in SELECT queries for aggregating data in corresponding order in MergeTree tables.

Possible values:

• 0 — GROUP BY optimization is disabled.
• 1 — GROUP BY optimization is enabled.

Default value: 0.

See Also

• GROUP BY optimization

mutations_sync

Allows to execute ALTER TABLE ... UPDATE|DELETE|MATERIALIZE INDEX|MATERIALIZE PROJECTION|MATERIALIZE COLUMN queries (mutations) synchronously.

Possible values:

• 0 - Mutations execute asynchronously.
• 1 - The query waits for all mutations to complete on the current server.
• 2 - The query waits for all mutations to complete on all replicas (if they exist).

Default value: 0.

See Also

• Synchronicity of ALTER Queries
• Mutations

ttl_only_drop_parts

Enables or disables complete dropping of data parts where all rows are expired in MergeTree tables.

When ttl_only_drop_parts is disabled (by default), the ClickHouse server only deletes expired rows according to their TTL.

When ttl_only_drop_parts is enabled, the ClickHouse server drops a whole part when all rows in it are expired.

Dropping whole parts instead of partial cleaning TTL-d rows allows having shorter merge_with_ttl_timeout times and lower impact on system performance.

Possible values:

• 0 — The complete dropping of data parts is disabled.
• 1 — The complete dropping of data parts is enabled.

Default value: 0.

See Also

• CREATE TABLE query clauses and settings (merge_with_ttl_timeout setting)
• Table TTL

lock_acquire_timeout

Defines how many seconds a locking request waits before failing.

Locking timeout is used to protect from deadlocks while executing read/write operations with tables. When the timeout expires and the locking request fails, the ClickHouse server throws an exception "Locking attempt timed out! Possible deadlock avoided. Client should retry." with error code DEADLOCK_AVOIDED.

Possible values:

• Positive integer (in seconds).
• 0 — No locking timeout.

Default value: 120 seconds.

cast_keep_nullable

Enables or disables keeping of the Nullable data type in CAST operations.

When the setting is enabled and the argument of CAST function is Nullable, the result is also transformed to Nullable type. When the setting is disabled, the result always has the destination type exactly.

Possible values:

• 0 — The CAST result has exactly the destination type specified.
• 1 — If the argument type is Nullable, the CAST result is transformed to Nullable(DestinationDataType).

Default value: 0.

Examples

The following query results in the destination data type exactly:

SET cast_keep_nullable = 0;
SELECT CAST(toNullable(toInt32(0)) AS Int32) as x, toTypeName(x);

Result:

┌─x─┬─toTypeName(CAST(toNullable(toInt32(0)), 'Int32'))─┐
│ 0 │ Int32                                             │
└───┴───────────────────────────────────────────────────┘

The following query results in the Nullable modification on the destination data type:

SET cast_keep_nullable = 1;
SELECT CAST(toNullable(toInt32(0)) AS Int32) as x, toTypeName(x);

Result:

┌─x─┬─toTypeName(CAST(toNullable(toInt32(0)), 'Int32'))─┐
│ 0 │ Nullable(Int32)                                   │
└───┴───────────────────────────────────────────────────┘

See Also

• CAST function

system_events_show_zero_values

Allows to select zero-valued events from system.events.

Some monitoring systems require passing all the metrics values to them for each checkpoint, even if the metric value is zero.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 0.

Examples

Query

SELECT * FROM system.events WHERE event='QueryMemoryLimitExceeded';

Result

Ok.

Query

SET system_events_show_zero_values = 1;
SELECT * FROM system.events WHERE event='QueryMemoryLimitExceeded';

Result

┌─event────────────────────┬─value─┬─description───────────────────────────────────────────┐
│ QueryMemoryLimitExceeded │     0 │ Number of times when memory limit exceeded for query. │
└──────────────────────────┴───────┴───────────────────────────────────────────────────────┘

allow_nullable_key

Allows using of the Nullable-typed values in a sorting and a primary key for MergeTree tables.

Possible values:

• 1 — Nullable-type expressions are allowed in keys.
• 0 — Nullable-type expressions are not allowed in keys.

Default value: 0.

:::note Nullable primary key usually indicates bad design. It is forbidden in almost all main stream DBMS. The feature is mainly for AggregatingMergeTree and is not heavily tested. Use with care. :::

:::note Do not enable this feature in version <= 21.8. It's not properly implemented and may lead to server crash. :::

aggregate_functions_null_for_empty

Enables or disables rewriting all aggregate functions in a query, adding -OrNull suffix to them. Enable it for SQL standard compatibility. It is implemented via query rewrite (similar to count_distinct_implementation setting) to get consistent results for distributed queries.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 0.

Example

Consider the following query with aggregate functions:

SELECT SUM(-1), MAX(0) FROM system.one WHERE 0;

With aggregate_functions_null_for_empty = 0 it would produce:

┌─SUM(-1)─┬─MAX(0)─┐
│       0 │      0 │
└─────────┴────────┘

With aggregate_functions_null_for_empty = 1 the result would be:

┌─SUMOrNull(-1)─┬─MAXOrNull(0)─┐
│          NULL │         NULL │
└───────────────┴──────────────┘

union_default_mode

Sets a mode for combining SELECT query results. The setting is only used when shared with UNION without explicitly specifying the UNION ALL or UNION DISTINCT.

Possible values:

• 'DISTINCT' — ClickHouse outputs rows as a result of combining queries removing duplicate rows.
• 'ALL' — ClickHouse outputs all rows as a result of combining queries including duplicate rows.
• '' — ClickHouse generates an exception when used with UNION.

Default value: ''.

See examples in UNION.

default_table_engine

Default table engine to use when ENGINE is not set in a CREATE statement.

Possible values:

• a string representing any valid table engine name

Default value: MergeTree.

Cloud default value: SharedMergeTree.

Example

Query:

SET default_table_engine = 'Log';

SELECT name, value, changed FROM system.settings WHERE name = 'default_table_engine';

Result:

┌─name─────────────────┬─value─┬─changed─┐
│ default_table_engine │ Log   │       1 │
└──────────────────────┴───────┴─────────┘

In this example, any new table that does not specify an Engine will use the Log table engine:

Query:

CREATE TABLE my_table (
    x UInt32,
    y UInt32
);

SHOW CREATE TABLE my_table;

Result:

┌─statement────────────────────────────────────────────────────────────────┐
│ CREATE TABLE default.my_table
(
    `x` UInt32,
    `y` UInt32
)
ENGINE = Log
└──────────────────────────────────────────────────────────────────────────┘

default_temporary_table_engine

Same as default_table_engine but for temporary tables.

Default value: Memory.

In this example, any new temporary table that does not specify an Engine will use the Log table engine:

Query:

SET default_temporary_table_engine = 'Log';

CREATE TEMPORARY TABLE my_table (
    x UInt32,
    y UInt32
);

SHOW CREATE TEMPORARY TABLE my_table;

Result:

┌─statement────────────────────────────────────────────────────────────────┐
│ CREATE TEMPORARY TABLE default.my_table
(
    `x` UInt32,
    `y` UInt32
)
ENGINE = Log
└──────────────────────────────────────────────────────────────────────────┘

data_type_default_nullable

Allows data types without explicit modifiers NULL or NOT NULL in column definition will be Nullable.

Possible values:

• 1 — The data types in column definitions are set to Nullable by default.
• 0 — The data types in column definitions are set to not Nullable by default.

Default value: 0.

mysql_map_string_to_text_in_show_columns

When enabled, String ClickHouse data type will be displayed as TEXT in SHOW COLUMNS.

Has an effect only when the connection is made through the MySQL wire protocol.

• 0 - Use BLOB.
• 1 - Use TEXT.

Default value: 1.

mysql_map_fixed_string_to_text_in_show_columns

When enabled, FixedString ClickHouse data type will be displayed as TEXT in SHOW COLUMNS.

Has an effect only when the connection is made through the MySQL wire protocol.

• 0 - Use BLOB.
• 1 - Use TEXT.

Default value: 1.

execute_merges_on_single_replica_time_threshold

Enables special logic to perform merges on replicas.

Possible values:

• Positive integer (in seconds).
• 0 — Special merges logic is not used. Merges happen in the usual way on all the replicas.

Default value: 0.

Usage

Selects one replica to perform the merge on. Sets the time threshold from the start of the merge. Other replicas wait for the merge to finish, then download the result. If the time threshold passes and the selected replica does not perform the merge, then the merge is performed on other replicas as usual.

High values for that threshold may lead to replication delays.

It can be useful when merges are CPU bounded not IO bounded (performing heavy data compression, calculating aggregate functions or default expressions that require a large amount of calculations, or just very high number of tiny merges).

max_final_threads

Sets the maximum number of parallel threads for the SELECT query data read phase with the FINAL modifier.

Possible values:

• Positive integer.
• 0 or 1 — Disabled. SELECT queries are executed in a single thread.

Default value: max_threads.

opentelemetry_start_trace_probability

Sets the probability that the ClickHouse can start a trace for executed queries (if no parent trace context is supplied).

Possible values:

• 0 — The trace for all executed queries is disabled (if no parent trace context is supplied).
• Positive floating-point number in the range [0..1]. For example, if the setting value is 0,5, ClickHouse can start a trace on average for half of the queries.
• 1 — The trace for all executed queries is enabled.

Default value: 0.

optimize_on_insert

Enables or disables data transformation before the insertion, as if merge was done on this block (according to table engine).

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 1.

Example

The difference between enabled and disabled:

Query:

SET optimize_on_insert = 1;

CREATE TABLE test1 (`FirstTable` UInt32) ENGINE = ReplacingMergeTree ORDER BY FirstTable;

INSERT INTO test1 SELECT number % 2 FROM numbers(5);

SELECT * FROM test1;

SET optimize_on_insert = 0;

CREATE TABLE test2 (`SecondTable` UInt32) ENGINE = ReplacingMergeTree ORDER BY SecondTable;

INSERT INTO test2 SELECT number % 2 FROM numbers(5);

SELECT * FROM test2;

Result:

┌─FirstTable─┐
│          0 │
│          1 │
└────────────┘

┌─SecondTable─┐
│           0 │
│           0 │
│           0 │
│           1 │
│           1 │
└─────────────┘

Note that this setting influences Materialized view and MaterializedMySQL behaviour.

engine_file_empty_if_not_exists

Allows to select data from a file engine table without file.

Possible values:

• 0 — SELECT throws exception.
• 1 — SELECT returns empty result.

Default value: 0.

engine_file_truncate_on_insert

Enables or disables truncate before insert in File engine tables.

Possible values:

• 0 — INSERT query appends new data to the end of the file.
• 1 — INSERT query replaces existing content of the file with the new data.

Default value: 0.

engine_file_allow_create_multiple_files

Enables or disables creating a new file on each insert in file engine tables if the format has the suffix (JSON, ORC, Parquet, etc.). If enabled, on each insert a new file will be created with a name following this pattern:

data.Parquet -> data.1.Parquet -> data.2.Parquet, etc.

Possible values:

• 0 — INSERT query appends new data to the end of the file.
• 1 — INSERT query creates a new file.

Default value: 0.

engine_file_skip_empty_files

Enables or disables skipping empty files in File engine tables.

Possible values:

• 0 — SELECT throws an exception if empty file is not compatible with requested format.
• 1 — SELECT returns empty result for empty file.

Default value: 0.

storage_file_read_method

Method of reading data from storage file, one of: read, pread, mmap. The mmap method does not apply to clickhouse-server (it's intended for clickhouse-local).

Default value: pread for clickhouse-server, mmap for clickhouse-local.

s3_truncate_on_insert

Enables or disables truncate before inserts in s3 engine tables. If disabled, an exception will be thrown on insert attempts if an S3 object already exists.

Possible values:

• 0 — INSERT query appends new data to the end of the file.
• 1 — INSERT query replaces existing content of the file with the new data.

Default value: 0.

s3_create_new_file_on_insert

Enables or disables creating a new file on each insert in s3 engine tables. If enabled, on each insert a new S3 object will be created with the key, similar to this pattern:

initial: data.Parquet.gz -> data.1.Parquet.gz -> data.2.Parquet.gz, etc.

Possible values:

• 0 — INSERT query appends new data to the end of the file.
• 1 — INSERT query creates a new file.

Default value: 0.

s3_skip_empty_files

Enables or disables skipping empty files in S3 engine tables.

Possible values:

• 0 — SELECT throws an exception if empty file is not compatible with requested format.
• 1 — SELECT returns empty result for empty file.

Default value: 0.

hdfs_truncate_on_insert

Enables or disables truncation before an insert in hdfs engine tables. If disabled, an exception will be thrown on an attempt to insert if a file in HDFS already exists.

Possible values:

• 0 — INSERT query appends new data to the end of the file.
• 1 — INSERT query replaces existing content of the file with the new data.

Default value: 0.

hdfs_create_new_file_on_insert {#hdfs_create_new_file_on_insert

Enables or disables creating a new file on each insert in HDFS engine tables. If enabled, on each insert a new HDFS file will be created with the name, similar to this pattern:

initial: data.Parquet.gz -> data.1.Parquet.gz -> data.2.Parquet.gz, etc.

Possible values:

• 0 — INSERT query appends new data to the end of the file.
• 1 — INSERT query creates a new file.

Default value: 0.

hdfs_skip_empty_files

Enables or disables skipping empty files in HDFS engine tables.

Possible values:

• 0 — SELECT throws an exception if empty file is not compatible with requested format.
• 1 — SELECT returns empty result for empty file.

Default value: 0.

engine_url_skip_empty_files

Enables or disables skipping empty files in URL engine tables.

Possible values:

• 0 — SELECT throws an exception if empty file is not compatible with requested format.
• 1 — SELECT returns empty result for empty file.

Default value: 0.

enable_url_encoding

Allows to enable/disable decoding/encoding path in uri in URL engine tables.

Enabled by default.

database_atomic_wait_for_drop_and_detach_synchronously

Adds a modifier SYNC to all DROP and DETACH queries.

Possible values:

• 0 — Queries will be executed with delay.
• 1 — Queries will be executed without delay.

Default value: 0.

show_table_uuid_in_table_create_query_if_not_nil

Sets the SHOW TABLE query display.

Possible values:

• 0 — The query will be displayed without table UUID.
• 1 — The query will be displayed with table UUID.

Default value: 0.

allow_experimental_live_view

Allows creation of a deprecated LIVE VIEW.

Possible values:

• 0 — Working with live views is disabled.
• 1 — Working with live views is enabled.

Default value: 0.

live_view_heartbeat_interval

Deprecated.

max_live_view_insert_blocks_before_refresh

Deprecated.

periodic_live_view_refresh

Deprecated.

http_connection_timeout

HTTP connection timeout (in seconds).

Possible values:

• Any positive integer.
• 0 - Disabled (infinite timeout).

Default value: 1.

http_send_timeout

HTTP send timeout (in seconds).

Possible values:

• Any positive integer.
• 0 - Disabled (infinite timeout).

Default value: 30.

http_receive_timeout

HTTP receive timeout (in seconds).

Possible values:

• Any positive integer.
• 0 - Disabled (infinite timeout).

Default value: 30.

check_query_single_value_result

Defines the level of detail for the CHECK TABLE query result for MergeTree family engines .

Possible values:

• 0 — the query shows a check status for every individual data part of a table.
• 1 — the query shows the general table check status.

Default value: 0.

prefer_column_name_to_alias

Enables or disables using the original column names instead of aliases in query expressions and clauses. It especially matters when alias is the same as the column name, see Expression Aliases. Enable this setting to make aliases syntax rules in ClickHouse more compatible with most other database engines.

Possible values:

• 0 — The column name is substituted with the alias.
• 1 — The column name is not substituted with the alias.

Default value: 0.

Example

The difference between enabled and disabled:

Query:

SET prefer_column_name_to_alias = 0;
SELECT avg(number) AS number, max(number) FROM numbers(10);

Result:

Received exception from server (version 21.5.1):
Code: 184. DB::Exception: Received from localhost:9000. DB::Exception: Aggregate function avg(number) is found inside another aggregate function in query: While processing avg(number) AS number.

Query:

SET prefer_column_name_to_alias = 1;
SELECT avg(number) AS number, max(number) FROM numbers(10);

Result:

┌─number─┬─max(number)─┐
│    4.5 │           9 │
└────────┴─────────────┘

limit

Sets the maximum number of rows to get from the query result. It adjusts the value set by the LIMIT clause, so that the limit, specified in the query, cannot exceed the limit, set by this setting.

Possible values:

• 0 — The number of rows is not limited.
• Positive integer.

Default value: 0.

offset

Sets the number of rows to skip before starting to return rows from the query. It adjusts the offset set by the OFFSET clause, so that these two values are summarized.

Possible values:

• 0 — No rows are skipped .
• Positive integer.

Default value: 0.

Example

Input table:

CREATE TABLE test (i UInt64) ENGINE = MergeTree() ORDER BY i;
INSERT INTO test SELECT number FROM numbers(500);

Query:

SET limit = 5;
SET offset = 7;
SELECT * FROM test LIMIT 10 OFFSET 100;

Result:

┌───i─┐
│ 107 │
│ 108 │
│ 109 │
└─────┘

optimize_syntax_fuse_functions

Enables to fuse aggregate functions with identical argument. It rewrites query contains at least two aggregate functions from sum, count or avg with identical argument to sumCount.

Possible values:

• 0 — Functions with identical argument are not fused.
• 1 — Functions with identical argument are fused.

Default value: 0.

Example

Query:

CREATE TABLE fuse_tbl(a Int8, b Int8) Engine = Log;
SET optimize_syntax_fuse_functions = 1;
EXPLAIN SYNTAX SELECT sum(a), sum(b), count(b), avg(b) from fuse_tbl FORMAT TSV;

Result:

SELECT
    sum(a),
    sumCount(b).1,
    sumCount(b).2,
    (sumCount(b).1) / (sumCount(b).2)
FROM fuse_tbl

optimize_rewrite_aggregate_function_with_if

Rewrite aggregate functions with if expression as argument when logically equivalent. For example, avg(if(cond, col, null)) can be rewritten to avgOrNullIf(cond, col). It may improve performance.

:::note Supported only with experimental analyzer (allow_experimental_analyzer = 1). :::

allow_experimental_database_replicated

Enables to create databases with Replicated engine.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 0.

Cloud default value: 1.

database_replicated_initial_query_timeout_sec

Sets how long initial DDL query should wait for Replicated database to process previous DDL queue entries in seconds.

Possible values:

• Positive integer.
• 0 — Unlimited.

Default value: 300.

distributed_ddl_task_timeout

Sets timeout for DDL query responses from all hosts in cluster. If a DDL request has not been performed on all hosts, a response will contain a timeout error and a request will be executed in an async mode. Negative value means infinite.

Possible values:

• Positive integer.
• 0 — Async mode.
• Negative integer — infinite timeout.

Default value: 180.

distributed_ddl_output_mode

Sets format of distributed DDL query result.

Possible values:

• throw — Returns result set with query execution status for all hosts where query is finished. If query has failed on some hosts, then it will rethrow the first exception. If query is not finished yet on some hosts and distributed_ddl_task_timeout exceeded, then it throws TIMEOUT_EXCEEDED exception.
• none — Is similar to throw, but distributed DDL query returns no result set.
• null_status_on_timeout — Returns NULL as execution status in some rows of result set instead of throwing TIMEOUT_EXCEEDED if query is not finished on the corresponding hosts.
• never_throw — Do not throw TIMEOUT_EXCEEDED and do not rethrow exceptions if query has failed on some hosts.
• none_only_active - similar to none, but doesn't wait for inactive replicas of the Replicated database. Note: with this mode it's impossible to figure out that the query was not executed on some replica and will be executed in background.
• null_status_on_timeout_only_active — similar to null_status_on_timeout, but doesn't wait for inactive replicas of the Replicated database
• throw_only_active — similar to throw, but doesn't wait for inactive replicas of the Replicated database

Default value: throw.

Cloud default value: none.

flatten_nested

Sets the data format of a nested columns.

Possible values:

• 1 — Nested column is flattened to separate arrays.
• 0 — Nested column stays a single array of tuples.

Default value: 1.

Usage

If the setting is set to 0, it is possible to use an arbitrary level of nesting.

Examples

Query:

SET flatten_nested = 1;
CREATE TABLE t_nest (`n` Nested(a UInt32, b UInt32)) ENGINE = MergeTree ORDER BY tuple();

SHOW CREATE TABLE t_nest;

Result:

┌─statement───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ CREATE TABLE default.t_nest
(
    `n.a` Array(UInt32),
    `n.b` Array(UInt32)
)
ENGINE = MergeTree
ORDER BY tuple()
SETTINGS index_granularity = 8192 │
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘

Query:

SET flatten_nested = 0;

CREATE TABLE t_nest (`n` Nested(a UInt32, b UInt32)) ENGINE = MergeTree ORDER BY tuple();

SHOW CREATE TABLE t_nest;

Result:

┌─statement──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ CREATE TABLE default.t_nest
(
    `n` Nested(a UInt32, b UInt32)
)
ENGINE = MergeTree
ORDER BY tuple()
SETTINGS index_granularity = 8192 │
└────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘

external_table_functions_use_nulls

Defines how mysql, postgresql and odbc table functions use Nullable columns.

Possible values:

• 0 — The table function explicitly uses Nullable columns.
• 1 — The table function implicitly uses Nullable columns.

Default value: 1.

Usage

If the setting is set to 0, the table function does not make Nullable columns and inserts default values instead of NULL. This is also applicable for NULL values inside arrays.

optimize_use_projections

Enables or disables projection optimization when processing SELECT queries.

Possible values:

• 0 — Projection optimization disabled.
• 1 — Projection optimization enabled.

Default value: 1.

force_optimize_projection

Enables or disables the obligatory use of projections in SELECT queries, when projection optimization is enabled (see optimize_use_projections setting).

Possible values:

• 0 — Projection optimization is not obligatory.
• 1 — Projection optimization is obligatory.

Default value: 0.

force_optimize_projection_name

If it is set to a non-empty string, check that this projection is used in the query at least once.

Possible values:

• string: name of projection that used in a query

Default value: ''.

preferred_optimize_projection_name

If it is set to a non-empty string, ClickHouse will try to apply specified projection in query.

Possible values:

• string: name of preferred projection

Default value: ''.

alter_sync

Allows to set up waiting for actions to be executed on replicas by ALTER, OPTIMIZE or TRUNCATE queries.

Possible values:

• 0 — Do not wait.
• 1 — Wait for own execution.
• 2 — Wait for everyone.

Default value: 1.

Cloud default value: 0.

:::note alter_sync is applicable to Replicated tables only, it does nothing to alters of not Replicated tables. :::

replication_wait_for_inactive_replica_timeout

Specifies how long (in seconds) to wait for inactive replicas to execute ALTER, OPTIMIZE or TRUNCATE queries.

Possible values:

• 0 — Do not wait.
• Negative integer — Wait for unlimited time.
• Positive integer — The number of seconds to wait.

Default value: 120 seconds.

regexp_max_matches_per_row

Sets the maximum number of matches for a single regular expression per row. Use it to protect against memory overload when using greedy regular expression in the extractAllGroupsHorizontal function.

Possible values:

• Positive integer.

Default value: 1000.

http_max_single_read_retries

Sets the maximum number of retries during a single HTTP read.

Possible values:

• Positive integer.

Default value: 1024.

log_queries_probability

Allows a user to write to query_log, query_thread_log, and query_views_log system tables only a sample of queries selected randomly with the specified probability. It helps to reduce the load with a large volume of queries in a second.

Possible values:

• 0 — Queries are not logged in the system tables.
• Positive floating-point number in the range [0..1]. For example, if the setting value is 0.5, about half of the queries are logged in the system tables.
• 1 — All queries are logged in the system tables.

Default value: 1.

short_circuit_function_evaluation

Allows calculating the if, multiIf, and, and or functions according to a short scheme. This helps optimize the execution of complex expressions in these functions and prevent possible exceptions (such as division by zero when it is not expected).

Possible values:

• enable — Enables short-circuit function evaluation for functions that are suitable for it (can throw an exception or computationally heavy).
• force_enable — Enables short-circuit function evaluation for all functions.
• disable — Disables short-circuit function evaluation.

Default value: enable.

max_hyperscan_regexp_length

Defines the maximum length for each regular expression in the hyperscan multi-match functions.

Possible values:

• Positive integer.
• 0 - The length is not limited.

Default value: 0.

Example

Query:

SELECT multiMatchAny('abcd', ['ab','bcd','c','d']) SETTINGS max_hyperscan_regexp_length = 3;

Result:

┌─multiMatchAny('abcd', ['ab', 'bcd', 'c', 'd'])─┐
│                                              1 │
└────────────────────────────────────────────────┘

Query:

SELECT multiMatchAny('abcd', ['ab','bcd','c','d']) SETTINGS max_hyperscan_regexp_length = 2;

Result:

Exception: Regexp length too large.

See Also

• max_hyperscan_regexp_total_length

max_hyperscan_regexp_total_length

Sets the maximum length total of all regular expressions in each hyperscan multi-match function.

Possible values:

• Positive integer.
• 0 - The length is not limited.

Default value: 0.

Example

Query:

SELECT multiMatchAny('abcd', ['a','b','c','d']) SETTINGS max_hyperscan_regexp_total_length = 5;

Result:

┌─multiMatchAny('abcd', ['a', 'b', 'c', 'd'])─┐
│                                           1 │
└─────────────────────────────────────────────┘

Query:

SELECT multiMatchAny('abcd', ['ab','bc','c','d']) SETTINGS max_hyperscan_regexp_total_length = 5;

Result:

Exception: Total regexp lengths too large.

See Also

• max_hyperscan_regexp_length

enable_positional_arguments

Enables or disables supporting positional arguments for GROUP BY, LIMIT BY, ORDER BY statements.

Possible values:

• 0 — Positional arguments aren't supported.
• 1 — Positional arguments are supported: column numbers can use instead of column names.

Default value: 1.

Example

Query:

CREATE TABLE positional_arguments(one Int, two Int, three Int) ENGINE=Memory();

INSERT INTO positional_arguments VALUES (10, 20, 30), (20, 20, 10), (30, 10, 20);

SELECT * FROM positional_arguments ORDER BY 2,3;

Result:

┌─one─┬─two─┬─three─┐
│  30 │  10 │   20  │
│  20 │  20 │   10  │
│  10 │  20 │   30  │
└─────┴─────┴───────┘

enable_order_by_all

Enables or disables sorting with ORDER BY ALL syntax, see ORDER BY.

Possible values:

• 0 — Disable ORDER BY ALL.
• 1 — Enable ORDER BY ALL.

Default value: 1.

Example

Query:

CREATE TABLE TAB(C1 Int, C2 Int, ALL Int) ENGINE=Memory();

INSERT INTO TAB VALUES (10, 20, 30), (20, 20, 10), (30, 10, 20);

SELECT * FROM TAB ORDER BY ALL; -- returns an error that ALL is ambiguous

SELECT * FROM TAB ORDER BY ALL SETTINGS enable_order_by_all = 0;

Result:

┌─C1─┬─C2─┬─ALL─┐
│ 20 │ 20 │  10 │
│ 30 │ 10 │  20 │
│ 10 │ 20 │  30 │
└────┴────┴─────┘

splitby_max_substrings_includes_remaining_string

Controls whether function splitBy*() with argument max_substrings > 0 will include the remaining string in the last element of the result array.

Possible values:

• 0 - The remaining string will not be included in the last element of the result array.
• 1 - The remaining string will be included in the last element of the result array. This is the behavior of Spark's split() function and Python's 'string.split()' method.

Default value: 0

enable_extended_results_for_datetime_functions

Enables or disables returning results of type:

• Date32 with extended range (compared to type Date) for functions toStartOfYear, toStartOfISOYear, toStartOfQuarter, toStartOfMonth, toLastDayOfMonth, toStartOfWeek, toLastDayOfWeek and toMonday.
• DateTime64 with extended range (compared to type DateTime) for functions toStartOfDay, toStartOfHour, toStartOfMinute, toStartOfFiveMinutes, toStartOfTenMinutes, toStartOfFifteenMinutes and timeSlot.

Possible values:

• 0 — Functions return Date or DateTime for all types of arguments.
• 1 — Functions return Date32 or DateTime64 for Date32 or DateTime64 arguments and Date or DateTime otherwise.

Default value: 0.

function_locate_has_mysql_compatible_argument_order

Controls the order of arguments in function locate.

Possible values:

• 0 — Function locate accepts arguments (haystack, needle[, start_pos]).
• 1 — Function locate accepts arguments (needle, haystack, [, start_pos]) (MySQL-compatible behavior)

Default value: 1.

date_time_overflow_behavior

Defines the behavior when Date, Date32, DateTime, DateTime64 or integers are converted into Date, Date32, DateTime or DateTime64 but the value cannot be represented in the result type.

Possible values:

• ignore — Silently ignore overflows. The result is random.
• throw — Throw an exception in case of conversion overflow.
• saturate — Silently saturate the result. If the value is smaller than the smallest value that can be represented by the target type, the result is chosen as the smallest representable value. If the value is bigger than the largest value that can be represented by the target type, the result is chosen as the largest representable value.

Default value: ignore.

first_day_of_week

The first day of the week assumed by toStartOfInterval function when using weeks as unit.

Possible values:

• Monday - Week starts on Monday
• Sunday - Week starts on Sunday

Default value: 'Monday'.

optimize_move_to_prewhere

Enables or disables automatic PREWHERE optimization in SELECT queries.

Works only for *MergeTree tables.

Possible values:

• 0 — Automatic PREWHERE optimization is disabled.
• 1 — Automatic PREWHERE optimization is enabled.

Default value: 1.

optimize_move_to_prewhere_if_final

Enables or disables automatic PREWHERE optimization in SELECT queries with FINAL modifier.

Works only for *MergeTree tables.

Possible values:

• 0 — Automatic PREWHERE optimization in SELECT queries with FINAL modifier is disabled.
• 1 — Automatic PREWHERE optimization in SELECT queries with FINAL modifier is enabled.

Default value: 0.

See Also

• optimize_move_to_prewhere setting

optimize_using_constraints

Use constraints for query optimization. The default is false.

Possible values:

• true, false

optimize_append_index

Use constraints in order to append index condition. The default is false.

Possible values:

• true, false

optimize_substitute_columns

Use constraints for column substitution. The default is false.

Possible values:

• true, false

describe_include_subcolumns

Enables describing subcolumns for a DESCRIBE query. For example, members of a Tuple or subcolumns of a Map, Nullable or an Array data type.

Possible values:

• 0 — Subcolumns are not included in DESCRIBE queries.
• 1 — Subcolumns are included in DESCRIBE queries.

Default value: 0.

Example

See an example for the DESCRIBE statement.

alter_partition_verbose_result

Enables or disables the display of information about the parts to which the manipulation operations with partitions and parts have been successfully applied. Applicable to ATTACH PARTITION|PART and to FREEZE PARTITION.

Possible values:

• 0 — disable verbosity.
• 1 — enable verbosity.

Default value: 0.

Example

CREATE TABLE test(a Int64, d Date, s String) ENGINE = MergeTree PARTITION BY toYYYYMM(d) ORDER BY a;
INSERT INTO test VALUES(1, '2021-01-01', '');
INSERT INTO test VALUES(1, '2021-01-01', '');
ALTER TABLE test DETACH PARTITION ID '202101';

ALTER TABLE test ATTACH PARTITION ID '202101' SETTINGS alter_partition_verbose_result = 1;

┌─command_type─────┬─partition_id─┬─part_name────┬─old_part_name─┐
│ ATTACH PARTITION │ 202101       │ 202101_7_7_0 │ 202101_5_5_0  │
│ ATTACH PARTITION │ 202101       │ 202101_8_8_0 │ 202101_6_6_0  │
└──────────────────┴──────────────┴──────────────┴───────────────┘

ALTER TABLE test FREEZE SETTINGS alter_partition_verbose_result = 1;

┌─command_type─┬─partition_id─┬─part_name────┬─backup_name─┬─backup_path───────────────────┬─part_backup_path────────────────────────────────────────────┐
│ FREEZE ALL   │ 202101       │ 202101_7_7_0 │ 8           │ /var/lib/clickhouse/shadow/8/ │ /var/lib/clickhouse/shadow/8/data/default/test/202101_7_7_0 │
│ FREEZE ALL   │ 202101       │ 202101_8_8_0 │ 8           │ /var/lib/clickhouse/shadow/8/ │ /var/lib/clickhouse/shadow/8/data/default/test/202101_8_8_0 │
└──────────────┴──────────────┴──────────────┴─────────────┴───────────────────────────────┴─────────────────────────────────────────────────────────────┘

min_bytes_to_use_mmap_io

This is an experimental setting. Sets the minimum amount of memory for reading large files without copying data from the kernel to userspace. Recommended threshold is about 64 MB, because mmap/munmap is slow. It makes sense only for large files and helps only if data reside in the page cache.

Possible values:

• Positive integer.
• 0 — Big files read with only copying data from kernel to userspace.

Default value: 0.

shutdown_wait_unfinished_queries

Enables or disables waiting unfinished queries when shutdown server.

Possible values:

• 0 — Disabled.
• 1 — Enabled. The wait time equal shutdown_wait_unfinished config.

Default value: 0.

shutdown_wait_unfinished

The waiting time in seconds for currently handled connections when shutdown server.

Default Value: 5.

memory_overcommit_ratio_denominator

It represents soft memory limit in case when hard limit is reached on user level. This value is used to compute overcommit ratio for the query. Zero means skip the query. Read more about memory overcommit.

Default value: 1GiB.

memory_usage_overcommit_max_wait_microseconds

Maximum time thread will wait for memory to be freed in the case of memory overcommit on a user level. If the timeout is reached and memory is not freed, an exception is thrown. Read more about memory overcommit.

Default value: 5000000.

memory_overcommit_ratio_denominator_for_user

It represents soft memory limit in case when hard limit is reached on global level. This value is used to compute overcommit ratio for the query. Zero means skip the query. Read more about memory overcommit.

Default value: 1GiB.

Schema Inference settings

See schema inference documentation for more details.

schema_inference_use_cache_for_file {schema_inference_use_cache_for_file}

Enable schemas cache for schema inference in file table function.

Default value: true.

schema_inference_use_cache_for_s3 {schema_inference_use_cache_for_s3}

Enable schemas cache for schema inference in s3 table function.

Default value: true.

schema_inference_use_cache_for_url {schema_inference_use_cache_for_url}

Enable schemas cache for schema inference in url table function.

Default value: true.

schema_inference_use_cache_for_hdfs {schema_inference_use_cache_for_hdfs}

Enable schemas cache for schema inference in hdfs table function.

Default value: true.

schema_inference_cache_require_modification_time_for_url

Use schema from cache for URL with last modification time validation (for urls with Last-Modified header). If this setting is enabled and URL doesn't have Last-Modified header, schema from cache won't be used.

Default value: true.

use_structure_from_insertion_table_in_table_functions {use_structure_from_insertion_table_in_table_functions}

Use structure from insertion table instead of schema inference from data.

Possible values:

• 0 - disabled
• 1 - enabled
• 2 - auto

Default value: 2.

schema_inference_mode {schema_inference_mode}

The mode of schema inference. Possible values: default and union. See schema inference modes section for more details.

Default value: default.

compatibility

The compatibility setting causes ClickHouse to use the default settings of a previous version of ClickHouse, where the previous version is provided as the setting.

If settings are set to non-default values, then those settings are honored (only settings that have not been modified are affected by the compatibility setting).

This setting takes a ClickHouse version number as a string, like 22.3, 22.8. An empty value means that this setting is disabled.

Disabled by default.

:::note In ClickHouse Cloud the compatibility setting must be set by ClickHouse Cloud support. Please open a case to have it set. :::

allow_settings_after_format_in_insert

Control whether SETTINGS after FORMAT in INSERT queries is allowed or not. It is not recommended to use this, since this may interpret part of SETTINGS as values.

Example:

INSERT INTO FUNCTION null('foo String') SETTINGS max_threads=1 VALUES ('bar');

But the following query will work only with allow_settings_after_format_in_insert:

SET allow_settings_after_format_in_insert=1;
INSERT INTO FUNCTION null('foo String') VALUES ('bar') SETTINGS max_threads=1;

Possible values:

• 0 — Disallow.
• 1 — Allow.

Default value: 0.

:::note Use this setting only for backward compatibility if your use cases depend on old syntax. :::

session_timezone

Sets the implicit time zone of the current session or query. The implicit time zone is the time zone applied to values of type DateTime/DateTime64 which have no explicitly specified time zone. The setting takes precedence over the globally configured (server-level) implicit time zone. A value of '' (empty string) means that the implicit time zone of the current session or query is equal to the server time zone.

You can use functions timeZone() and serverTimeZone() to get the session time zone and server time zone.

Possible values:

• Any time zone name from system.time_zones, e.g. Europe/Berlin, UTC or Zulu

Default value: ''.

Examples:

SELECT timeZone(), serverTimeZone() FORMAT TSV

Europe/Berlin	Europe/Berlin

SELECT timeZone(), serverTimeZone() SETTINGS session_timezone = 'Asia/Novosibirsk' FORMAT TSV

Asia/Novosibirsk	Europe/Berlin

Assign session time zone 'America/Denver' to the inner DateTime without explicitly specified time zone:

SELECT toDateTime64(toDateTime64('1999-12-12 23:23:23.123', 3), 3, 'Europe/Zurich') SETTINGS session_timezone = 'America/Denver' FORMAT TSV

1999-12-13 07:23:23.123

:::warning Not all functions that parse DateTime/DateTime64 respect session_timezone. This can lead to subtle errors. See the following example and explanation. :::

CREATE TABLE test_tz (`d` DateTime('UTC')) ENGINE = Memory AS SELECT toDateTime('2000-01-01 00:00:00', 'UTC');

SELECT *, timeZone() FROM test_tz WHERE d = toDateTime('2000-01-01 00:00:00') SETTINGS session_timezone = 'Asia/Novosibirsk'
0 rows in set.

SELECT *, timeZone() FROM test_tz WHERE d = '2000-01-01 00:00:00' SETTINGS session_timezone = 'Asia/Novosibirsk'
┌───────────────────d─┬─timeZone()───────┐
│ 2000-01-01 00:00:00 │ Asia/Novosibirsk │
└─────────────────────┴──────────────────┘

This happens due to different parsing pipelines:

• toDateTime() without explicitly given time zone used in the first SELECT query honors setting session_timezone and the global time zone.
• In the second query, a DateTime is parsed from a String, and inherits the type and time zone of the existing columnd. Thus, setting session_timezone and the global time zone are not honored.

See also

• timezone

final

Automatically applies FINAL modifier to all tables in a query, to tables where FINAL is applicable, including joined tables and tables in sub-queries, and distributed tables.

Possible values:

• 0 - disabled
• 1 - enabled

Default value: 0.

Example:

CREATE TABLE test
(
    key Int64,
    some String
)
ENGINE = ReplacingMergeTree
ORDER BY key;

INSERT INTO test FORMAT Values (1, 'first');
INSERT INTO test FORMAT Values (1, 'second');

SELECT * FROM test;
┌─key─┬─some───┐
│   1 │ second │
└─────┴────────┘
┌─key─┬─some──┐
│   1 │ first │
└─────┴───────┘

SELECT * FROM test SETTINGS final = 1;
┌─key─┬─some───┐
│   1 │ second │
└─────┴────────┘

SET final = 1;
SELECT * FROM test;
┌─key─┬─some───┐
│   1 │ second │
└─────┴────────┘

asterisk_include_materialized_columns

Include MATERIALIZED columns for wildcard query (SELECT *).

Possible values:

• 0 - disabled
• 1 - enabled

Default value: 0.

asterisk_include_alias_columns

Include ALIAS columns for wildcard query (SELECT *).

Possible values:

• 0 - disabled
• 1 - enabled

Default value: 0.

async_socket_for_remote

Enables asynchronous read from socket while executing remote query.

Enabled by default.

async_query_sending_for_remote

Enables asynchronous connection creation and query sending while executing remote query.

Enabled by default.

use_hedged_requests

Enables hedged requests logic for remote queries. It allows to establish many connections with different replicas for query. New connection is enabled in case existent connection(s) with replica(s) were not established within hedged_connection_timeout or no data was received within receive_data_timeout. Query uses the first connection which send non empty progress packet (or data packet, if allow_changing_replica_until_first_data_packet); other connections are cancelled. Queries with max_parallel_replicas > 1 are supported.

Enabled by default.

Disabled by default on Cloud.

hedged_connection_timeout

If we can't establish connection with replica after this timeout in hedged requests, we start working with the next replica without cancelling connection to the previous. Timeout value is in milliseconds.

Default value: 50.

receive_data_timeout

This timeout is set when the query is sent to the replica in hedged requests, if we don't receive first packet of data and we don't make any progress in query execution after this timeout, we start working with the next replica, without cancelling connection to the previous. Timeout value is in milliseconds.

Default value: 2000

allow_changing_replica_until_first_data_packet

If it's enabled, in hedged requests we can start new connection until receiving first data packet even if we have already made some progress (but progress haven't updated for receive_data_timeout timeout), otherwise we disable changing replica after the first time we made progress.

parallel_view_processing

Enables pushing to attached views concurrently instead of sequentially.

Default value: false.

partial_result_on_first_cancel

When set to true and the user wants to interrupt a query (for example using Ctrl+C on the client), then the query continues execution only on data that was already read from the table. Afterwards, it will return a partial result of the query for the part of the table that was read. To fully stop the execution of a query without a partial result, the user should send 2 cancel requests.

Example without setting on Ctrl+C

SELECT sum(number) FROM numbers(10000000000)

Cancelling query.
Ok.
Query was cancelled.

0 rows in set. Elapsed: 1.334 sec. Processed 52.65 million rows, 421.23 MB (39.48 million rows/s., 315.85 MB/s.)

Example with setting on Ctrl+C

SELECT sum(number) FROM numbers(10000000000) SETTINGS partial_result_on_first_cancel=true

┌──────sum(number)─┐
│ 1355411451286266 │
└──────────────────┘

1 row in set. Elapsed: 1.331 sec. Processed 52.13 million rows, 417.05 MB (39.17 million rows/s., 313.33 MB/s.)

Possible values: true, false

Default value: false

function_json_value_return_type_allow_nullable

Control whether allow to return NULL when value is not exist for JSON_VALUE function.

SELECT JSON_VALUE('{"hello":"world"}', '$.b') settings function_json_value_return_type_allow_nullable=true;

┌─JSON_VALUE('{"hello":"world"}', '$.b')─┐
│ ᴺᵁᴸᴸ                                   │
└────────────────────────────────────────┘

1 row in set. Elapsed: 0.001 sec.

Possible values:

• true — Allow.
• false — Disallow.

Default value: false.

rename_files_after_processing

• Type: String

• Default value: Empty string

This setting allows to specify renaming pattern for files processed by file table function. When option is set, all files read by file table function will be renamed according to specified pattern with placeholders, only if files processing was successful.

Placeholders

• %a — Full original filename (e.g., "sample.csv").
• %f — Original filename without extension (e.g., "sample").
• %e — Original file extension with dot (e.g., ".csv").
• %t — Timestamp (in microseconds).
• %% — Percentage sign ("%").

Example

• Option: --rename_files_after_processing="processed_%f_%t%e"

• Query: SELECT * FROM file('sample.csv')

If reading sample.csv is successful, file will be renamed to processed_sample_1683473210851438.csv

function_json_value_return_type_allow_complex

Control whether allow to return complex type (such as: struct, array, map) for json_value function.

SELECT JSON_VALUE('{"hello":{"world":"!"}}', '$.hello') settings function_json_value_return_type_allow_complex=true

┌─JSON_VALUE('{"hello":{"world":"!"}}', '$.hello')─┐
│ {"world":"!"}                                    │
└──────────────────────────────────────────────────┘

1 row in set. Elapsed: 0.001 sec.

Possible values:

• true — Allow.
• false — Disallow.

Default value: false.

zstd_window_log_max

Allows you to select the max window log of ZSTD (it will not be used for MergeTree family)

Type: Int64

Default: 0

enable_deflate_qpl_codec

If turned on, the DEFLATE_QPL codec may be used to compress columns.

Possible values:

• 0 - Disabled
• 1 - Enabled

Type: Bool

enable_zstd_qat_codec

If turned on, the ZSTD_QAT codec may be used to compress columns.

Possible values:

• 0 - Disabled
• 1 - Enabled

Type: Bool

output_format_compression_level

Default compression level if query output is compressed. The setting is applied when SELECT query has INTO OUTFILE or when writing to table functions file, url, hdfs, s3, or azureBlobStorage.

Possible values: from 1 to 22

Default: 3

output_format_compression_zstd_window_log

Can be used when the output compression method is zstd. If greater than 0, this setting explicitly sets compression window size (power of 2) and enables a long-range mode for zstd compression. This can help to achieve a better compression ratio.

Possible values: non-negative numbers. Note that if the value is too small or too big, zstdlib will throw an exception. Typical values are from 20 (window size = 1MB) to 30 (window size = 1GB).

Default: 0

rewrite_count_distinct_if_with_count_distinct_implementation

Allows you to rewrite countDistcintIf with count_distinct_implementation setting.

Possible values:

• true — Allow.
• false — Disallow.

Default value: false.

precise_float_parsing

Switches Float32/Float64 parsing algorithms:

• If the value is 1, then precise method is used. It is slower than fast method, but it always returns a number that is the closest machine representable number to the input.
• Otherwise, fast method is used (default). It usually returns the same value as precise, but in rare cases result may differ by one or two least significant digits.

Possible values: 0, 1.

Default value: 0.

Example:

SELECT toFloat64('1.7091'), toFloat64('1.5008753E7') SETTINGS precise_float_parsing = 0;

┌─toFloat64('1.7091')─┬─toFloat64('1.5008753E7')─┐
│  1.7090999999999998 │       15008753.000000002 │
└─────────────────────┴──────────────────────────┘

SELECT toFloat64('1.7091'), toFloat64('1.5008753E7') SETTINGS precise_float_parsing = 1;

┌─toFloat64('1.7091')─┬─toFloat64('1.5008753E7')─┐
│              1.7091 │                 15008753 │
└─────────────────────┴──────────────────────────┘

validate_tcp_client_information

Determines whether validation of client information enabled when query packet is received from a client using a TCP connection.

If true, an exception will be thrown on invalid client information from the TCP client.

If false, the data will not be validated. The server will work with clients of all versions.

The default value is false.

Example

<validate_tcp_client_information>true</validate_tcp_client_information>

print_pretty_type_names

Allows to print deep-nested type names in a pretty way with indents in DESCRIBE query and in toTypeName() function.

Example:

CREATE TABLE test (a Tuple(b String, c Tuple(d Nullable(UInt64), e Array(UInt32), f Array(Tuple(g String, h Map(String, Array(Tuple(i String, j UInt64))))), k Date), l Nullable(String))) ENGINE=Memory;
DESCRIBE TABLE test FORMAT TSVRaw SETTINGS print_pretty_type_names=1;

a	Tuple(
    b String,
    c Tuple(
        d Nullable(UInt64),
        e Array(UInt32),
        f Array(Tuple(
            g String,
            h Map(
                String,
                Array(Tuple(
                    i String,
                    j UInt64
                ))
            )
        )),
        k Date
    ),
    l Nullable(String)
)

allow_experimental_statistic

Allows defining columns with statistics and manipulate statistics.

allow_statistic_optimize

Allows using statistic to optimize the order of prewhere conditions.

analyze_index_with_space_filling_curves

If a table has a space-filling curve in its index, e.g. ORDER BY mortonEncode(x, y), and the query has conditions on its arguments, e.g. x >= 10 AND x <= 20 AND y >= 20 AND y <= 30, use the space-filling curve for index analysis.

query_plan_enable_optimizations

Toggles query optimization at the query plan level.

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Possible values:

• 0 - Disable all optimizations at the query plan level
• 1 - Enable optimizations at the query plan level (but individual optimizations may still be disabled via their individual settings)

Default value: 1.

query_plan_max_optimizations_to_apply

Limits the total number of optimizations applied to query plan, see setting query_plan_enable_optimizations. Useful to avoid long optimization times for complex queries. If the actual number of optimizations exceeds this setting, an exception is thrown.

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Type: UInt64.

Default value: '10000'

query_plan_lift_up_array_join

Toggles a query-plan-level optimization which moves ARRAY JOINs up in the execution plan. Only takes effect if setting query_plan_enable_optimizations is 1.

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Possible values:

• 0 - Disable
• 1 - Enable

Default value: 1.

query_plan_push_down_limit

Toggles a query-plan-level optimization which moves LIMITs down in the execution plan. Only takes effect if setting query_plan_enable_optimizations is 1.

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Possible values:

• 0 - Disable
• 1 - Enable

Default value: 1.

query_plan_split_filter

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Toggles a query-plan-level optimization which splits filters into expressions. Only takes effect if setting query_plan_enable_optimizations is 1.

Possible values:

• 0 - Disable
• 1 - Enable

Default value: 1.

query_plan_merge_expressions

Toggles a query-plan-level optimization which merges consecutive filters. Only takes effect if setting query_plan_enable_optimizations is 1.

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Possible values:

• 0 - Disable
• 1 - Enable

Default value: 1.

query_plan_filter_push_down

Toggles a query-plan-level optimization which moves filters down in the execution plan. Only takes effect if setting query_plan_enable_optimizations is 1.

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Possible values:

• 0 - Disable
• 1 - Enable

Default value: 1.

query_plan_execute_functions_after_sorting

Toggles a query-plan-level optimization which moves expressions after sorting steps. Only takes effect if setting query_plan_enable_optimizations is 1.

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Possible values:

• 0 - Disable
• 1 - Enable

Default value: 1.

query_plan_reuse_storage_ordering_for_window_functions

Toggles a query-plan-level optimization which uses storage sorting when sorting for window functions. Only takes effect if setting query_plan_enable_optimizations is 1.

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Possible values:

• 0 - Disable
• 1 - Enable

Default value: 1.

query_plan_lift_up_union

Toggles a query-plan-level optimization which moves larger subtrees of the query plan into union to enable further optimizations. Only takes effect if setting query_plan_enable_optimizations is 1.

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Possible values:

• 0 - Disable
• 1 - Enable

Default value: 1.

query_plan_distinct_in_order

Toggles the distinct in-order optimization query-plan-level optimization. Only takes effect if setting query_plan_enable_optimizations is 1.

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Possible values:

• 0 - Disable
• 1 - Enable

Default value: 1.

query_plan_read_in_order

Toggles the read in-order optimization query-plan-level optimization. Only takes effect if setting query_plan_enable_optimizations is 1.

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Possible values:

• 0 - Disable
• 1 - Enable

Default value: 1.

query_plan_aggregation_in_order

Toggles the aggregation in-order query-plan-level optimization. Only takes effect if setting query_plan_enable_optimizations is 1.

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Possible values:

• 0 - Disable
• 1 - Enable

Default value: 0.

query_plan_remove_redundant_sorting

Toggles a query-plan-level optimization which removes redundant sorting steps, e.g. in subqueries. Only takes effect if setting query_plan_enable_optimizations is 1.

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Possible values:

• 0 - Disable
• 1 - Enable

Default value: 1.

query_plan_remove_redundant_distinct

Toggles a query-plan-level optimization which removes redundant DISTINCT steps. Only takes effect if setting query_plan_enable_optimizations is 1.

:::note This is an expert-level setting which should only be used for debugging by developers. The setting may change in future in backward-incompatible ways or be removed. :::

Possible values:

• 0 - Disable
• 1 - Enable

Default value: 1.

dictionary_use_async_executor

Execute a pipeline for reading dictionary source in several threads. It's supported only by dictionaries with local CLICKHOUSE source.

You may specify it in SETTINGS section of dictionary definition:

CREATE DICTIONARY t1_dict ( key String, attr UInt64 )
PRIMARY KEY key
SOURCE(CLICKHOUSE(QUERY `SELECT key, attr FROM t1 GROUP BY key`))
LIFETIME(MIN 0 MAX 3600)
LAYOUT(COMPLEX_KEY_HASHED_ARRAY())
SETTINGS(dictionary_use_async_executor=1, max_threads=8);

storage_metadata_write_full_object_key

When set to true the metadata files are written with VERSION_FULL_OBJECT_KEY format version. With that format full object storage key names are written to the metadata files. When set to false the metadata files are written with the previous format version, VERSION_INLINE_DATA. With that format only suffixes of object storage key names are written to the metadata files. The prefix for all of object storage key names is set in configurations files at storage_configuration.disks section.

Default value: false.

s3_use_adaptive_timeouts

When set to true than for all s3 requests first two attempts are made with low send and receive timeouts. When set to false than all attempts are made with identical timeouts.

Default value: true.

allow_experimental_variant_type

Allows creation of experimental Variant.

Default value: false.

use_variant_as_common_type

Allows to use Variant type as a result type for if/multiIf/array/map functions when there is no common type for argument types.

Example:

SET use_variant_as_common_type = 1;
SELECT toTypeName(if(number % 2, number, range(number))) as variant_type FROM numbers(1);
SELECT if(number % 2, number, range(number)) as variant FROM numbers(5);

┌─variant_type───────────────────┐
│ Variant(Array(UInt64), UInt64) │
└────────────────────────────────┘
┌─variant───┐
│ []        │
│ 1         │
│ [0,1]     │
│ 3         │
│ [0,1,2,3] │
└───────────┘

SET use_variant_as_common_type = 1;
SELECT toTypeName(multiIf((number % 4) = 0, 42, (number % 4) = 1, [1, 2, 3], (number % 4) = 2, 'Hello, World!', NULL)) AS variant_type FROM numbers(1);
SELECT multiIf((number % 4) = 0, 42, (number % 4) = 1, [1, 2, 3], (number % 4) = 2, 'Hello, World!', NULL) AS variant FROM numbers(4);

─variant_type─────────────────────────┐
│ Variant(Array(UInt8), String, UInt8) │
└──────────────────────────────────────┘

┌─variant───────┐
│ 42            │
│ [1,2,3]       │
│ Hello, World! │
│ ᴺᵁᴸᴸ          │
└───────────────┘

SET use_variant_as_common_type = 1;
SELECT toTypeName(array(range(number), number, 'str_' || toString(number))) as array_of_variants_type from numbers(1);
SELECT array(range(number), number, 'str_' || toString(number)) as array_of_variants FROM numbers(3);

┌─array_of_variants_type────────────────────────┐
│ Array(Variant(Array(UInt64), String, UInt64)) │
└───────────────────────────────────────────────┘

┌─array_of_variants─┐
│ [[],0,'str_0']    │
│ [[0],1,'str_1']   │
│ [[0,1],2,'str_2'] │
└───────────────────┘

SET use_variant_as_common_type = 1;
SELECT toTypeName(map('a', range(number), 'b', number, 'c', 'str_' || toString(number))) as map_of_variants_type from numbers(1);
SELECT map('a', range(number), 'b', number, 'c', 'str_' || toString(number)) as map_of_variants FROM numbers(3);

┌─map_of_variants_type────────────────────────────────┐
│ Map(String, Variant(Array(UInt64), String, UInt64)) │
└─────────────────────────────────────────────────────┘

┌─map_of_variants───────────────┐
│ {'a':[],'b':0,'c':'str_0'}    │
│ {'a':[0],'b':1,'c':'str_1'}   │
│ {'a':[0,1],'b':2,'c':'str_2'} │
└───────────────────────────────┘

Default value: false.

default_normal_view_sql_security

Allows to set default SQL SECURITY option while creating a normal view. More about SQL security.

The default value is INVOKER.

default_materialized_view_sql_security

Allows to set a default value for SQL SECURITY option when creating a materialized view. More about SQL security.

The default value is DEFINER.

default_view_definer

Allows to set default DEFINER option while creating a view. More about SQL security.

The default value is CURRENT_USER.

max_partition_size_to_drop

Restriction on dropping partitions in query time. The value 0 means that you can drop partitions without any restrictions.

Default value: 50 GB.

Cloud default value: 1 TB.

:::note This query setting overwrites its server setting equivalent, see max_partition_size_to_drop :::

max_table_size_to_drop

Restriction on deleting tables in query time. The value 0 means that you can delete all tables without any restrictions.

Default value: 50 GB.

Cloud default value: 1 TB.

:::note This query setting overwrites its server setting equivalent, see max_table_size_to_drop :::

iceberg_engine_ignore_schema_evolution

Allow to ignore schema evolution in Iceberg table engine and read all data using schema specified by the user on table creation or latest schema parsed from metadata on table creation.

:::note Enabling this setting can lead to incorrect result as in case of evolved schema all data files will be read using the same schema. :::

Default value: 'false'.

allow_suspicious_primary_key

Allow suspicious PRIMARY KEY/ORDER BY for MergeTree (i.e. SimpleAggregateFunction).

mysql_datatypes_support_level

Defines how MySQL types are converted to corresponding ClickHouse types. A comma separated list in any combination of decimal, datetime64, date2Date32 or date2String.

• decimal: convert NUMERIC and DECIMAL types to Decimal when precision allows it.
• datetime64: convert DATETIME and TIMESTAMP types to DateTime64 instead of DateTime when precision is not 0.
• date2Date32: convert DATE to Date32 instead of Date. Takes precedence over date2String.
• date2String: convert DATE to String instead of Date. Overridden by datetime64.