Contains information about the columns in all the tables.
You can use this table to get information similar to the [DESCRIBE TABLE](../query_language/misc.md#misc-describe-table) query, but for multiple tables at once.
The `system.columns` table contains the following columns (the type of the corresponding column is shown in brackets):
-`database` (String) — Database name.
-`table` (String) — Table name.
-`name` (String) — Column name.
-`type` (String) — Column type.
-`default_kind` (String) — Expression type (`DEFAULT`, `MATERIALIZED`, `ALIAS`) for the default value, or an empty string if it is not defined.
-`default_expression` (String) — Expression for the default value, or an empty string if it is not defined.
-`data_compressed_bytes` (UInt64) — The size of compressed data, in bytes.
-`data_uncompressed_bytes` (UInt64) — The size of decompressed data, in bytes.
-`marks_bytes` (UInt64) — The size of marks, in bytes.
-`comment` (String) — The comment about column, or an empty string if it is not defined.
-`is_in_partition_key` (UInt8) — Flag that indicates whether the column is in partition expression.
-`is_in_sorting_key` (UInt8) — Flag that indicates whether the column is in sorting key expression.
-`is_in_primary_key` (UInt8) — Flag that indicates whether the column is in primary key expression.
-`is_in_sampling_key` (UInt8) — Flag that indicates whether the column is in sampling key expression.
Note that the amount of memory used by the dictionary is not proportional to the number of items stored in it. So for flat and cached dictionaries, all the memory cells are pre-assigned, regardless of how full the dictionary actually is.
Contains information about the number of events that have occurred in the system. For example, in the table, you can find how many `SELECT` queries were processed since the ClickHouse server started.
│ Query │ 12 │ Number of queries to be interpreted and potentially executed. Does not include queries that failed to parse or were rejected due to AST size limits, quota limits or limits on the number of simultaneously running queries. May include internal queries initiated by ClickHouse itself. Does not count subqueries. │
│ ReadBufferFromFileDescriptorReadBytes │ 9931 │ Number of bytes read from file descriptors. If the file is compressed, this will show the compressed data size. │
Contains information about parameters [graphite_rollup](server_settings/settings.md#server_settings-graphite_rollup) which use in tables with [\*GraphiteMergeTree](table_engines/graphitemergetree.md) engines.
Contains metrics which can be calculated instantly, or have a current value. For example, the number of simultaneously processed queries or the current replica delay. This table is always up to date.
│ BackgroundPoolTask │ 0 │ Number of active tasks in BackgroundProcessingPool (merges, mutations, fetches, or replication queue bookkeeping) │
│ BackgroundSchedulePoolTask │ 0 │ Number of active tasks in BackgroundSchedulePool. This pool is used for periodic ReplicatedMergeTree tasks, like cleaning old data parts, altering data parts, replica re-initialization, etc. │
│ DiskSpaceReservedForMerge │ 0 │ Disk space reserved for currently running background merges. It is slightly more than the total size of currently merging parts. │
│ DistributedSend │ 0 │ Number of connections to remote servers sending data that was INSERTed into Distributed tables. Both synchronous and asynchronous mode. │
-`partition` (`String`) – The partition name. To learn what a partition is, see the description of the [ALTER](../query_language/alter.md#query_language_queries_alter) query.
-`active` (`UInt8`) – Flag that indicates whether the part is active. If a part is active, it is used in a table; otherwise, it will be deleted. Inactive data parts remain after merging.
-`marks` (`UInt64`) – The number of marks. To get the approximate number of rows in a data part, multiply `marks` by the index granularity (usually 8192).
-`data_compressed_bytes` (`UInt64`) – Total size of compressed data in the data part. All the auxiliary files (for example, files with marks) are not included.
-`data_uncompressed_bytes` (`UInt64`) – Total size of uncompressed data in the data part. All the auxiliary files (for example, files with marks) are not included.
-`modification_time` (`DateTime`) – The modification time of the directory with the data part. This usually corresponds to the time of data part creation.|
-`refcount` (`UInt32`) – The number of places where the data part is used. A value greater than 2 indicates that the data part is used in queries or merges.
-`data_version` (`UInt64`) – Block number that is used to determine which mutations should be applied to the data part (the mutations with the bigger version than `data_version`).
-`primary_key_bytes_in_memory` (`UInt64`) – The amount of memory (in bytes) used by primary key values.
-`primary_key_bytes_in_memory_allocated` (`UInt64`) – The amount of memory (in bytes) reserved for primary key values.
-`is_frozen` (`UInt8`) – Flag that shows partition data backup existence. 1, the backup exists. 0, the backup doesn't exist. For more details, see [FREEZE PARTITION](../query_language/alter.md#alter_freeze-partition)
-`database` (`String`) – Name of the database.
-`table` (`String`) – Name of the table.
-`engine` (`String`) – Name of the table engine without parameters.
-`path` (`String`) – Absolute path to the folder with data part files.
-`uncompressed_hash_of_compressed_files` (`String`) – [sipHash128](../query_language/functions/hash_functions.md#hash_functions-siphash128) of the file with marks.
The `system.part_log` table is created only if the [part_log](server_settings/settings.md#server_settings-part-log) server setting is specified.
This table contains information about the events that occurred with the [data parts](table_engines/custom_partitioning_key.md) in the [MergeTree](table_engines/mergetree.md) family tables. For instance, adding or merging data.
The `system.part_log` table contains the following columns:
-`event_type` (Enum) — Type of the event that occurred with the data part. Can have one of the following values: `NEW_PART` — inserting, `MERGE_PARTS` — merging, `DOWNLOAD_PART` — downloading, `REMOVE_PART` — removing or detaching using [DETACH PARTITION](../query_language/alter.md#alter_detach-partition), `MUTATE_PART` — updating.
-`event_date` (Date) — Event date.
-`event_time` (DateTime) — Event time.
-`duration_ms` (UInt64) — Duration.
-`database` (String) — Name of the database the data part is in.
-`table` (String) — Name of the table the data part is in.
-`part_name` (String) — Name of the data part.
-`partition_id` (String) — ID of the partition that the data part was inserted to. The column takes the 'all' value if the partitioning is by `tuple()`.
-`rows` (UInt64) — The number of rows in the data part.
-`size_in_bytes` (UInt64) — Size of the data part in bytes.
-`merged_from` (Array(String)) — An array of names of the parts which the current part was made up from (after the merge).
-`bytes_uncompressed` (UInt64) — Size of uncompressed bytes.
-`read_rows` (UInt64) — The number of rows was read during the merge.
-`read_bytes` (UInt64) — The number of bytes was read during the merge.
-`error` (UInt16) — The code number of the occurred error.
-`exception` (String) — Text message of the occurred error.
The `system.part_log` table is created after the first inserting data to the `MergeTree` table.
user String – Name of the user who made the request. For distributed query processing, this is the user who helped the requestor server send the query to this server, not the user who made the distributed request on the requestor server.
rows_read UInt64 - The number of rows read from the table. For distributed processing, on the requestor server, this is the total for all remote servers.
bytes_read UInt64 - The number of uncompressed bytes read from the table. For distributed processing, on the requestor server, this is the total for all remote servers.
total_rows_approx UInt64 - The approximation of the total number of rows that should be read. For distributed processing, on the requestor server, this is the total for all remote servers. It can be updated during request processing, when new sources to process become known.
Contains information about execution of queries. For each query, you can see processing start time, duration of processing, error messages and other information.
ClickHouse creates this table only if the [query_log](server_settings/settings.md#server_settings-query-log) server parameter is specified. This parameter sets the logging rules, such as the logging interval or the name of the table the queries will be logged in.
To enable query logging, set the [log_queries](settings/settings.md#settings-log-queries) parameter to 1. For details, see the [Settings](settings/settings.md) section.
2. Child queries that were initiated by other queries (for distributed query execution). For these types of queries, information about the parent queries is shown in the `initial_*` columns.
-`Settings.Names` (Array(String)) — Names of settings that were changed when the client ran the query. To enable logging changes to settings, set the `log_query_settings` parameter to 1.
By default, logs are added to the table at intervals of 7.5 seconds. You can set this interval in the [query_log](server_settings/settings.md#server_settings-query-log) server setting (see the `flush_interval_milliseconds` parameter). To flush the logs forcibly from the memory buffer into the table, use the `SYSTEM FLUSH LOGS` query.
The storage period for logs is unlimited. Logs aren't automatically deleted from the table. You need to organize the removal of outdated logs yourself.
You can specify an arbitrary partitioning key for the `system.query_log` table in the [query_log](server_settings/settings.md#server_settings-query-log) server setting (see the `partition_by` parameter).
This mode is turned on if the config doesn't have sections with ZooKeeper, if an unknown error occurred when reinitializing sessions in ZooKeeper, and during session reinitialization in ZooKeeper.
The table contains information about [mutations](../query_language/alter.md#alter-mutations) of MergeTree tables and their progress. Each mutation command is represented by a single row. The table has the following columns:
**database**, **table** - The name of the database and table to which the mutation was applied.
**mutation_id** - The ID of the mutation. For replicated tables these IDs correspond to znode names in the `<table_path_in_zookeeper>/mutations/` directory in ZooKeeper. For unreplicated tables the IDs correspond to file names in the data directory of the table.
**command** - The mutation command string (the part of the query after `ALTER TABLE [db.]table`).
**create_time** - When this mutation command was submitted for execution.
**block_numbers.partition_id**, **block_numbers.number** - A Nested column. For mutations of replicated tables contains one record for each partition: the partition ID and the block number that was acquired by the mutation (in each partition only parts that contain blocks with numbers less than the block number acquired by the mutation in that partition will be mutated). Because in non-replicated tables blocks numbers in all partitions form a single sequence, for mutatations of non-replicated tables the column will contain one record with a single block number acquired by the mutation.
**parts_to_do** - The number of data parts that need to be mutated for the mutation to finish.
**is_done** - Is the mutation done? Note that even if `parts_to_do = 0` it is possible that a mutation of a replicated table is not done yet because of a long-running INSERT that will create a new data part that will need to be mutated.
If there were problems with mutating some parts the following columns contain additional information:
**latest_failed_part** - The name of the most recent part that could not be mutated.
**latest_fail_time** - The time of the most recent part mutation failure.
**latest_fail_reason** - The exception message that caused the most recent part mutation failure.
