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

Settings

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.

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Forces a query to an out-of-date replica if updated data is not available.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ClickHouse selects the most relevant from the outdated replicas of the table.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.

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.

Works with tables in the MergeTree family.

format_schema

This parameter is useful when you are using formats that require a schema definition, such as Cap’n Proto or Protobuf. The value depends on the format.

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.

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.

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.

input_format_allow_errors_num

Sets the maximum number of acceptable errors when reading from text formats (CSV, TSV, etc.).

The default value is 0.

Always pair it with input_format_allow_errors_ratio.

If an error occurred while reading rows but the error counter is still less than input_format_allow_errors_num, ClickHouse ignores the row and moves on to the next one.

If both input_format_allow_errors_num and input_format_allow_errors_ratio are exceeded, ClickHouse throws an exception.

input_format_allow_errors_ratio

Sets the maximum percentage of errors allowed when reading from text formats (CSV, TSV, etc.). The percentage of errors is set as a floating-point number between 0 and 1.

The default value is 0.

Always pair it with input_format_allow_errors_num.

If an error occurred while reading rows but the error counter is still less than input_format_allow_errors_ratio, ClickHouse ignores the row and moves on to the next one.

If both input_format_allow_errors_num and input_format_allow_errors_ratio are exceeded, ClickHouse throws an exception.

input_format_values_interpret_expressions

Enables or disables the full SQL parser if the fast stream parser can’t parse the data. This setting is used only for the Values format at the data insertion. For more information about syntax parsing, see the Syntax section.

Possible values:

• 0 — Disabled.

  In this case, you must provide formatted data. See the Formats section.

• 1 — Enabled.

  In this case, you can use an SQL expression as a value, but data insertion is much slower this way. If you insert only formatted data, then ClickHouse behaves as if the setting value is 0.

Default value: 1.

Example of Use

Insert the DateTime type value with the different settings.

SET input_format_values_interpret_expressions = 0;
INSERT INTO datetime_t VALUES (now())

Exception on client:
Code: 27. DB::Exception: Cannot parse input: expected ) before: now()): (at row 1)

SET input_format_values_interpret_expressions = 1;
INSERT INTO datetime_t VALUES (now())

Ok.

The last query is equivalent to the following:

SET input_format_values_interpret_expressions = 0;
INSERT INTO datetime_t SELECT now()

Ok.

input_format_values_deduce_templates_of_expressions

Enables or disables template deduction for SQL expressions in Values format. It allows parsing and interpreting expressions in Values much faster if expressions in consecutive rows have the same structure. ClickHouse tries to deduce the template of an expression, parse the following rows using this template and evaluate the expression on a batch of successfully parsed rows.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 1.

For the following query:

INSERT INTO test VALUES (lower('Hello')), (lower('world')), (lower('INSERT')), (upper('Values')), ...

• If input_format_values_interpret_expressions=1 and format_values_deduce_templates_of_expressions=0, expressions are interpreted separately for each row (this is very slow for large number of rows).
• If input_format_values_interpret_expressions=0 and format_values_deduce_templates_of_expressions=1, expressions in the first, second and third rows are parsed using template lower(String) and interpreted together, expression in the forth row is parsed with another template (upper(String)).
• If input_format_values_interpret_expressions=1 and format_values_deduce_templates_of_expressions=1, the same as in previous case, but also allows fallback to interpreting expressions separately if it’s not possible to deduce template.

input_format_values_accurate_types_of_literals

This setting is used only when input_format_values_deduce_templates_of_expressions = 1. Expressions for some column may have the same structure, but contain numeric literals of different types, e.g.

(..., abs(0), ...),             -- UInt64 literal
(..., abs(3.141592654), ...),   -- Float64 literal
(..., abs(-1), ...),            -- Int64 literal

Possible values:

• 0 — Disabled.

  In this case, ClickHouse may use a more general type for some literals (e.g., Float64 or Int64 instead of UInt64 for 42), but it may cause overflow and precision issues.

• 1 — Enabled.

  In this case, ClickHouse checks the actual type of literal and uses an expression template of the corresponding type. In some cases, it may significantly slow down expression evaluation in Values.

Default value: 1.

input_format_defaults_for_omitted_fields

When performing INSERT queries, replace omitted input column values with default values of the respective columns. This option only applies to JSONEachRow, CSV and TabSeparated formats.

!!! note "Note" When this option is enabled, extended table metadata are sent from server to client. It consumes additional computing resources on the server and can reduce performance.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 1.

input_format_tsv_empty_as_default

When enabled, replace empty input fields in TSV with default values. For complex default expressions input_format_defaults_for_omitted_fields must be enabled too.

Disabled by default.

input_format_tsv_enum_as_number

Enables or disables parsing enum values as enum ids for TSV input format.

Possible values:

• 0 — Enum values are parsed as values.
• 1 — Enum values are parsed as enum IDs.

Default value: 0.

Example

Consider the table:

CREATE TABLE table_with_enum_column_for_tsv_insert (Id Int32,Value Enum('first' = 1, 'second' = 2)) ENGINE=Memory();

When the input_format_tsv_enum_as_number setting is enabled:

SET input_format_tsv_enum_as_number = 1;
INSERT INTO table_with_enum_column_for_tsv_insert FORMAT TSV 102	2;
INSERT INTO table_with_enum_column_for_tsv_insert FORMAT TSV 103	1;
SELECT * FROM table_with_enum_column_for_tsv_insert;

Result:

┌──Id─┬─Value──┐
│ 102 │ second │
└─────┴────────┘
┌──Id─┬─Value──┐
│ 103 │ first  │
└─────┴────────┘

When the input_format_tsv_enum_as_number setting is disabled, the INSERT query:

SET input_format_tsv_enum_as_number = 0;
INSERT INTO table_with_enum_column_for_tsv_insert FORMAT TSV 102	2;

throws an exception.

input_format_null_as_default

Enables or disables using default values if input data contain NULL, but the data type of the corresponding column in not Nullable(T) (for text input formats).

input_format_skip_unknown_fields

Enables or disables skipping insertion of extra data.

When writing data, ClickHouse throws an exception if input data contain columns that do not exist in the target table. If skipping is enabled, ClickHouse doesn’t insert extra data and doesn’t throw an exception.

Supported formats:

• JSONEachRow
• CSVWithNames
• TabSeparatedWithNames
• TSKV

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 0.

input_format_import_nested_json

Enables or disables the insertion of JSON data with nested objects.

Supported formats:

• JSONEachRow

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 0.

See also:

• Usage of Nested Structures with the JSONEachRow format.

input_format_with_names_use_header

Enables or disables checking the column order when inserting data.

To improve insert performance, we recommend disabling this check if you are sure that the column order of the input data is the same as in the target table.

Supported formats:

• CSVWithNames
• TabSeparatedWithNames

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 1.

date_time_input_format

Allows choosing a parser of the text representation of date and time.

The setting doesn’t apply to date and time functions.

Possible values:

• 'best_effort' — Enables extended parsing.

  ClickHouse can parse the basic YYYY-MM-DD HH:MM:SS format and all ISO 8601 date and time formats. For example, '2018-06-08T01:02:03.000Z'.

• 'basic' — Use basic parser.

  ClickHouse can parse only the basic YYYY-MM-DD HH:MM:SS or YYYY-MM-DD format. For example, 2019-08-20 10:18:56 or 2019-08-20.

Default value: 'basic'.

See also:

• DateTime data type.
• Functions for working with dates and times.

date_time_output_format

Allows choosing different output formats of the text representation of date and time.

Possible values:

• simple - Simple output format.

  Clickhouse output date and time YYYY-MM-DD hh:mm:ss format. For example, 2019-08-20 10:18:56. The calculation is performed according to the data type's time zone (if present) or server time zone.

• iso - ISO output format.

  Clickhouse output date and time in ISO 8601 YYYY-MM-DDThh:mm:ssZ format. For example, 2019-08-20T10:18:56Z. Note that output is in UTC (Z means UTC).

• unix_timestamp - Unix timestamp output format.

  Clickhouse output date and time in Unix timestamp format. For example 1566285536.

Default value: simple.

See also:

• DateTime data type.
• Functions for working with dates and times.

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_any_take_last_row

Changes behaviour of join operations with ANY strictness.

!!! warning "Attention" 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.

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 used and the less disk I/O 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 "Warning" Use this setting only for backward compatibility if your use cases depend on legacy JOIN behaviour.

When the legacy behaviour 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 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

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 (sets of column parts). The internal processing cycles for a single block are efficient enough, but there are noticeable expenditures on each block. The max_block_size setting is a recommendation for what size of the block (in a count of rows) to load from tables. The block size shouldn’t be too small, so that the expenditures on each block are still noticeable, but not too large so that the query with LIMIT that is completed after the first block is processed quickly. The goal is 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: 65,536.

Blocks the size of max_block_size are not always loaded from the table. If it is obvious that less data needs to be retrieved, a smaller block is processed.

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.

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:

• Any 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:

• Any 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 doesn’t 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 doesn’t 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 value:

• 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 then 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.

Queries’ threads runned by ClickHouse with this setup are logged according to the rules in the query_thread_log server configuration parameter.

Example:

log_query_threads=1

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 length 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; │
└─────────────┴───────────┘

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 doesn’t affect the size of the inserted blocks. The setting also doesn’t 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 because 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 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: 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 lags more than the set value, this replica is not used.

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.

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 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 "Warning" 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 "Warning" 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 part of a query that can be taken to RAM for parsing with the SQL parser. The INSERT query also contains data for INSERT that is processed by a separate stream parser (that consumes O(1) RAM), which is not included in this restriction.

Default value: 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 cancelled and sending the progress.

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

connect_timeout, receive_timeout, send_timeout

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

Default value: 10, 300, 300.

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

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.

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: 50.

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

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 (don’t 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.

Yandex.Metrica uses this parameter set 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.

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
• 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.yandex.ru are different in one position, while example01-01-1 and example01-02-2 differ in two places. This method might seem primitive, but it doesn’t require external data about network topology, and it doesn’t 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.

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.

!!! warning "Warning" Disable this setting if you use max_parallel_replicas.

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 setting is useful for replicated tables with a sampling 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 doesn't 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.

!!! warning "Warning" 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.

compile

Enable compilation of queries. By default, 0 (disabled).

The compilation is only used for part of the query-processing pipeline: for the first stage of aggregation (GROUP BY). If this portion of the pipeline was compiled, the query may run faster due to the deployment of short cycles and inlining aggregate function calls. The maximum performance improvement (up to four times faster in rare cases) is seen for queries with multiple simple aggregate functions. Typically, the performance gain is insignificant. In very rare cases, it may slow down query execution.

min_count_to_compile

How many times to potentially use a compiled chunk of code before running compilation. By default, 3. For testing, the value can be set to 0: compilation runs synchronously and the query waits for the end of the compilation process before continuing execution. For all other cases, use values ​​starting with 1. Compilation normally takes about 5-10 seconds. If the value is 1 or more, compilation occurs asynchronously in a separate thread. The result will be used as soon as it is ready, including queries that are currently running.

Compiled code is required for each different combination of aggregate functions used in the query and the type of keys in the GROUP BY clause. The results of the compilation are saved in the build directory in the form of .so files. There is no restriction on the number of compilation results since they don’t use very much space. Old results will be used after server restarts, except in the case of a server upgrade – in this case, the old results are deleted.

output_format_json_quote_64bit_integers

If the value is true, integers appear in quotes when using JSON* Int64 and UInt64 formats (for compatibility with most JavaScript implementations); otherwise, integers are output without the quotes.

output_format_json_quote_denormals

Enables +nan, -nan, +inf, -inf outputs in JSON output format.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 0.

Example

Consider the following table account_orders:

┌─id─┬─name───┬─duration─┬─period─┬─area─┐
│  1 │ Andrew │       20 │      0 │  400 │
│  2 │ John   │       40 │      0 │    0 │
│  3 │ Bob    │       15 │      0 │ -100 │
└────┴────────┴──────────┴────────┴──────┘

When output_format_json_quote_denormals = 0, the query returns null values in output:

SELECT area/period FROM account_orders FORMAT JSON;

{
        "meta":
        [
                {
                        "name": "divide(area, period)",
                        "type": "Float64"
                }
        ],

        "data":
        [
                {
                        "divide(area, period)": null
                },
                {
                        "divide(area, period)": null
                },
                {
                        "divide(area, period)": null
                }
        ],

        "rows": 3,

        "statistics":
        {
                "elapsed": 0.003648093,
                "rows_read": 3,
                "bytes_read": 24
        }
}

When output_format_json_quote_denormals = 1, the query returns:

{
        "meta":
        [
                {
                        "name": "divide(area, period)",
                        "type": "Float64"
                }
        ],

        "data":
        [
                {
                        "divide(area, period)": "inf"
                },
                {
                        "divide(area, period)": "-nan"
                },
                {
                        "divide(area, period)": "-inf"
                }
        ],

        "rows": 3,

        "statistics":
        {
                "elapsed": 0.000070241,
                "rows_read": 3,
                "bytes_read": 24
        }
}

format_csv_delimiter

The character is interpreted as a delimiter in the CSV data. By default, the delimiter is ,.

input_format_csv_unquoted_null_literal_as_null

For CSV input format enables or disables parsing of unquoted NULL as literal (synonym for \N).

input_format_csv_enum_as_number

Enables or disables parsing enum values as enum ids for CSV input format.

Possible values:

• 0 — Enum values are parsed as values.
• 1 — Enum values are parsed as enum IDs.

Default value: 0.

Examples

Consider the table:

CREATE TABLE table_with_enum_column_for_csv_insert (Id Int32,Value Enum('first' = 1, 'second' = 2)) ENGINE=Memory();

When the input_format_csv_enum_as_number setting is enabled:

SET input_format_csv_enum_as_number = 1;
INSERT INTO table_with_enum_column_for_csv_insert FORMAT CSV 102,2;
SELECT * FROM table_with_enum_column_for_csv_insert;

Result:

┌──Id─┬─Value─────┐
│ 102 │ second    │
└─────┴───────────┘

When the input_format_csv_enum_as_number setting is disabled, the INSERT query:

SET input_format_csv_enum_as_number = 0;
INSERT INTO table_with_enum_column_for_csv_insert FORMAT CSV 102,2;

throws an exception.

output_format_csv_crlf_end_of_line

Use DOS/Windows-style line separator (CRLF) in CSV instead of Unix style (LF).

output_format_tsv_crlf_end_of_line

Use DOC/Windows-style line separator (CRLF) in TSV instead of Unix style (LF).

insert_quorum

Enables the quorum writes.

• If insert_quorum < 2, the quorum writes are disabled.
• If insert_quorum >= 2, the quorum writes are enabled.

Default value: 0.

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.

All the replicas in the quorum are consistent, i.e., they contain data from all previous INSERT queries. The INSERT sequence is linearized.

When reading the data written from the insert_quorum, you can use the select_sequential_consistency option.

ClickHouse generates an exception

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

See also:

• insert_quorum_timeout
• 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
• select_sequential_consistency

select_sequential_consistency

Enables or disables sequential consistency for SELECT queries:

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.

See also:

• insert_quorum
• insert_quorum_timeout

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).

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 Zookeeper) 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.

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 doesn’t 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 completelly processed on the shard, initiator only proxy the data).
• 2 - Same as 1 but apply ORDER BY and LIMIT on the initiator (can be used for queries with ORDER BY and/or LIMIT).

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 │
└───────┘

Default value: 0

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 does nothing).

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 doesn’t 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
• optimize_skip_unused_shards

!!! note "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.

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_directory_monitor_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_directory_monitor_max_sleep_time_ms

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

Possible values:

• A positive integer number of milliseconds.

Default value: 30000 milliseconds (30 seconds).

distributed_directory_monitor_batch_inserts

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.

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.

!!! warning "Warning" 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 don’t 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).

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.

See also:

• System table trace_log

allow_introspection_functions

Enables or disables introspections 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

• Type: bool
• Default value: True

Enable order-preserving parallel parsing of data formats. Supported only for TSV, TKSV, CSV, and JSONEachRow formats.

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.

output_format_avro_codec

Sets the compression codec used for output Avro file.

Type: string

Possible values:

• null — No compression
• deflate — Compress with Deflate (zlib)
• snappy — Compress with Snappy

Default value: snappy (if available) or deflate.

output_format_avro_sync_interval

Sets minimum data size (in bytes) between synchronization markers for output Avro file.

Type: unsigned int

Possible values: 32 (32 bytes) - 1073741824 (1 GiB)

Default value: 32768 (32 KiB)

format_avro_schema_registry_url

Sets Confluent Schema Registry URL to use with AvroConfluent format.

Default value: Empty.

input_format_avro_allow_missing_fields

Enables using fields that are not specified in Avro or AvroConfluent format schema. When a field is not found in the schema, ClickHouse uses the default value instead of throwing an exception.

Possible values:

• 0 — Disabled.
• 1 — Enabled.

Default value: 0.

background_pool_size

Sets the number of threads performing background operations in table engines (for example, merges in MergeTree engine tables). This setting is applied from the default profile at the ClickHouse server start and can’t be changed in a user session. By adjusting this setting, you manage CPU and disk load. Smaller pool size utilizes less CPU and disk resources, but background processes advance slower which might eventually impact query performance.

Before changing it, please also take a look at related MergeTree settings, such as number_of_free_entries_in_pool_to_lower_max_size_of_merge and number_of_free_entries_in_pool_to_execute_mutation.

Possible values:

• Any positive integer.

Default value: 16.

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.

insert_distributed_sync

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 asynchronous mode. When insert_distributed_sync=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 asynchronous mode.
• 1 — Data is inserted in synchronous mode.

Default value: 0.

See Also

• Distributed Table Engine
• Managing Distributed Tables

insert_distributed_one_random_shard

Enables or disables random shard insertion into a Distributed table when there is no distributed key.

By default, when inserting data into a Distributed table with more than one shard, the ClickHouse server will reject any insertion request if there is no distributed key. When insert_distributed_one_random_shard = 1, insertions are allowed and data is forwarded randomly among all shards.

Possible values:

• 0 — Insertion is rejected if there are multiple shards and no distributed key is given.
• 1 — Insertion is done randomly among all available shards when no distributed key is given.

Default value: 0.

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 async (insert_distributed_sync) 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 "Note" - with use_compact_format_in_distributed_parts_names=0 changes from cluster definition will not be applied for async 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

Sets the number of threads performing background flush in Buffer-engine tables. This setting is applied at the ClickHouse server start and can’t be changed in a user session.

Possible values:

• Any positive integer.

Default value: 16.

background_move_pool_size

Sets the number of threads performing background moves of data parts for MergeTree-engine tables. This setting is applied at the ClickHouse server start and can’t be changed in a user session.

Possible values:

• Any positive integer.

Default value: 8.

background_schedule_pool_size

Sets the number of threads performing background tasks for replicated tables, Kafka streaming, DNS cache updates. This setting is applied at ClickHouse server start and can’t be changed in a user session.

Possible values:

• Any positive integer.

Default value: 16.

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 don’t 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

Sets the number of threads performing background tasks for distributed sends. This setting is applied at the ClickHouse server start and can’t be changed in a user session.

Possible values:

• Any positive integer.

Default value: 16.

background_message_broker_schedule_pool_size

Sets the number of threads performing background tasks for message streaming. This setting is applied at the ClickHouse server start and can’t be changed in a user session.

Possible values:

• Any positive integer.

Default value: 16.

See Also

• Kafka engine.
• RabbitMQ engine.

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 don’t 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

output_format_pretty_grid_charset

Allows changing a charset which is used for printing grids borders. Available charsets are UTF-8, ASCII.

Example

SET output_format_pretty_grid_charset = 'UTF-8';
SELECT * FROM a;
┌─a─┐
│ 1 │
└───┘

SET output_format_pretty_grid_charset = 'ASCII';
SELECT * FROM a;
+-a-+
| 1 |
+---+

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 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

output_format_pretty_max_value_width

Limits the width of value displayed in Pretty formats. If the value width exceeds the limit, the value is cut.

Possible values:

• Positive integer.
• 0 — The value is cut completely.

Default value: 10000 symbols.

Examples

Query:

SET output_format_pretty_max_value_width = 10;
SELECT range(number) FROM system.numbers LIMIT 10 FORMAT PrettyCompactNoEscapes;

Result:

┌─range(number)─┐
│ []            │
│ [0]           │
│ [0,1]         │
│ [0,1,2]       │
│ [0,1,2,3]     │
│ [0,1,2,3,4⋯   │
│ [0,1,2,3,4⋯   │
│ [0,1,2,3,4⋯   │
│ [0,1,2,3,4⋯   │
│ [0,1,2,3,4⋯   │
└───────────────┘

Query with zero width:

SET output_format_pretty_max_value_width = 0;
SELECT range(number) FROM system.numbers LIMIT 5 FORMAT PrettyCompactNoEscapes;

Result:

┌─range(number)─┐
│ ⋯             │
│ ⋯             │
│ ⋯             │
│ ⋯             │
│ ⋯             │
└───────────────┘

output_format_pretty_row_numbers

Adds row numbers to output in the Pretty format.

Possible values:

• 0 — Output without row numbers.
• 1 — Output with row numbers.

Default value: 0.

Example

Query:

SET output_format_pretty_row_numbers = 1;
SELECT TOP 3 name, value FROM system.settings;

Result:

   ┌─name────────────────────┬─value───┐
1. │ min_compress_block_size │ 65536   │
2. │ max_compress_block_size │ 1048576 │
3. │ max_block_size          │ 65505   │
   └─────────────────────────┴─────────┘

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_experimental_bigint_types

Enables or disables integer values exceeding the range that is supported by the int data type.

Possible values:

• 1 — The bigint data type is enabled.
• 0 — The bigint data type is disabled.

Default value: 0.

persistent

Disables persistency for the Set and Join table engines.

Reduces the I/O overhead. Suitable for scenarios that pursue performance and do not require persistence.

Possible values:

• 1 — Enabled.
• 0 — Disabled.

Default value: 1.

output_format_tsv_null_representation

Defines the representation of NULL for TSV output format. User can set any string as a value, for example, My NULL.

Default value: \N.

Examples

Query

SELECT * FROM tsv_custom_null FORMAT TSV;

Result

788
\N
\N

Query

SET output_format_tsv_null_representation = 'My NULL';
SELECT * FROM tsv_custom_null FORMAT TSV;

Result

788
My NULL
My NULL

output_format_json_array_of_rows

Enables the ability to output all rows as a JSON array in the JSONEachRow format.

Possible values:

• 1 — ClickHouse outputs all rows as an array, each row in the JSONEachRow format.
• 0 — ClickHouse outputs each row separately in the JSONEachRow format.

Default value: 0.

Example of a query with the enabled setting

Query:

SET output_format_json_array_of_rows = 1;
SELECT number FROM numbers(3) FORMAT JSONEachRow;

Result:

[
{"number":"0"},
{"number":"1"},
{"number":"2"}
]

Example of a query with the disabled setting

Query:

SET output_format_json_array_of_rows = 0;
SELECT number FROM numbers(3) FORMAT JSONEachRow;

Result:

{"number":"0"}
{"number":"1"}
{"number":"2"}

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.

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.

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.

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: 16.

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 MaterializeMySQL 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 replaces existing content of the file with the new data.

Default value: 0.

allow_experimental_geo_types

Allows working with experimental geo data types.

Possible values:

• 0 — Working with geo data types is disabled.
• 1 — Working with geo data types is enabled.

Default value: 0.

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 experimental live views.

Possible values:

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

Default value: 0.

live_view_heartbeat_interval

Sets the heartbeat interval in seconds to indicate live view is alive .

Default value: 15.

max_live_view_insert_blocks_before_refresh

Sets the maximum number of inserted blocks after which mergeable blocks are dropped and query for live view is re-executed.

Default value: 64.

temporary_live_view_timeout

Sets the interval in seconds after which live view with timeout is deleted.

Default value: 5.

periodic_live_view_refresh

Sets the interval in seconds after which periodically refreshed live view is forced to refresh.

Default value: 60.

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 │
└─────┘

Original article