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Miscellaneous Queries

ATTACH

This query is exactly the same as CREATE, but

  • Instead of the word CREATE it uses the word ATTACH.
  • The query does not create data on the disk, but assumes that data is already in the appropriate places, and just adds information about the table to the server. After executing an ATTACH query, the server will know about the existence of the table.

If the table was previously detached (DETACH), meaning that its structure is known, you can use shorthand without defining the structure.

ATTACH TABLE [IF NOT EXISTS] [db.]name [ON CLUSTER cluster]

This query is used when starting the server. The server stores table metadata as files with ATTACH queries, which it simply runs at launch (with the exception of system tables, which are explicitly created on the server).

CHECK TABLE

Checks if the data in the table is corrupted.

CHECK TABLE [db.]name

The CHECK TABLE query compares actual file sizes with the expected values which are stored on the server. If the file sizes do not match the stored values, it means the data is corrupted. This can be caused, for example, by a system crash during query execution.

The query response contains the result column with a single row. The row has a value of Boolean type:

  • 0 - The data in the table is corrupted.
  • 1 - The data maintains integrity.

The CHECK TABLE query supports the following table engines:

Performed over the tables with another table engines causes an exception.

Engines from the *Log family don’t provide automatic data recovery on failure. Use the CHECK TABLE query to track data loss in a timely manner.

For MergeTree family engines, the CHECK TABLE query shows a check status for every individual data part of a table on the local server.

If the data is corrupted

If the table is corrupted, you can copy the non-corrupted data to another table. To do this:

  1. Create a new table with the same structure as damaged table. To do this execute the query CREATE TABLE <new_table_name> AS <damaged_table_name>.
  2. Set the max_threads value to 1 to process the next query in a single thread. To do this run the query SET max_threads = 1.
  3. Execute the query INSERT INTO <new_table_name> SELECT * FROM <damaged_table_name>. This request copies the non-corrupted data from the damaged table to another table. Only the data before the corrupted part will be copied.
  4. Restart the clickhouse-client to reset the max_threads value.

DESCRIBE TABLE

DESC|DESCRIBE TABLE [db.]table [INTO OUTFILE filename] [FORMAT format]

Returns the following String type columns:

  • name — Column name.
  • type— Column type.
  • default_type — Clause that is used in default expression (DEFAULT, MATERIALIZED or ALIAS). Column contains an empty string, if the default expression isn’t specified.
  • default_expression — Value specified in the DEFAULT clause.
  • comment_expression — Comment text.

Nested data structures are output in “expanded” format. Each column is shown separately, with the name after a dot.

DETACH

Deletes information about the ‘name’ table from the server. The server stops knowing about the table’s existence.

DETACH TABLE [IF EXISTS] [db.]name [ON CLUSTER cluster]

This does not delete the table’s data or metadata. On the next server launch, the server will read the metadata and find out about the table again. Similarly, a “detached” table can be re-attached using the ATTACH query (with the exception of system tables, which do not have metadata stored for them).

There is no DETACH DATABASE query.

DROP

This query has two types: DROP DATABASE and DROP TABLE.

DROP DATABASE [IF EXISTS] db [ON CLUSTER cluster]

Deletes all tables inside the ‘db’ database, then deletes the ‘db’ database itself. If IF EXISTS is specified, it doesn’t return an error if the database doesn’t exist.

DROP [TEMPORARY] TABLE [IF EXISTS] [db.]name [ON CLUSTER cluster]

Deletes the table. If IF EXISTS is specified, it doesn’t return an error if the table doesn’t exist or the database doesn’t exist.

DROP DICTIONARY [IF EXISTS] [db.]name

Delets the dictionary. If IF EXISTS is specified, it doesn’t return an error if the table doesn’t exist or the database doesn’t exist.

EXISTS

EXISTS [TEMPORARY] [TABLE|DICTIONARY] [db.]name [INTO OUTFILE filename] [FORMAT format]

Returns a single UInt8-type column, which contains the single value 0 if the table or database doesn’t exist, or 1 if the table exists in the specified database.

KILL QUERY

KILL QUERY [ON CLUSTER cluster]
  WHERE <where expression to SELECT FROM system.processes query>
  [SYNC|ASYNC|TEST]
  [FORMAT format]

Attempts to forcibly terminate the currently running queries. The queries to terminate are selected from the system.processes table using the criteria defined in the WHERE clause of the KILL query.

Examples:

-- Forcibly terminates all queries with the specified query_id:
KILL QUERY WHERE query_id='2-857d-4a57-9ee0-327da5d60a90'

-- Synchronously terminates all queries run by 'username':
KILL QUERY WHERE user='username' SYNC

Read-only users can only stop their own queries.

By default, the asynchronous version of queries is used (ASYNC), which doesn’t wait for confirmation that queries have stopped.

The synchronous version (SYNC) waits for all queries to stop and displays information about each process as it stops. The response contains the kill_status column, which can take the following values:

  1. ‘finished’ – The query was terminated successfully.
  2. ‘waiting’ – Waiting for the query to end after sending it a signal to terminate.
  3. The other values ​​explain why the query can’t be stopped.

A test query (TEST) only checks the user’s rights and displays a list of queries to stop.

KILL MUTATION

KILL MUTATION [ON CLUSTER cluster]
  WHERE <where expression to SELECT FROM system.mutations query>
  [TEST]
  [FORMAT format]

Tries to cancel and remove mutations that are currently executing. Mutations to cancel are selected from the system.mutations table using the filter specified by the WHERE clause of the KILL query.

A test query (TEST) only checks the user’s rights and displays a list of queries to stop.

Examples:

-- Cancel and remove all mutations of the single table:
KILL MUTATION WHERE database = 'default' AND table = 'table'

-- Cancel the specific mutation:
KILL MUTATION WHERE database = 'default' AND table = 'table' AND mutation_id = 'mutation_3.txt'

The query is useful when a mutation is stuck and cannot finish (e.g. if some function in the mutation query throws an exception when applied to the data contained in the table).

Changes already made by the mutation are not rolled back.

OPTIMIZE

OPTIMIZE TABLE [db.]name [ON CLUSTER cluster] [PARTITION partition | PARTITION ID 'partition_id'] [FINAL] [DEDUPLICATE]

This query tries to initialize an unscheduled merge of data parts for tables with a table engine from the MergeTree family.

The OPTMIZE query is also supported for the MaterializedView and the Buffer engines. Other table engines aren’t supported.

When OPTIMIZE is used with the ReplicatedMergeTree family of table engines, ClickHouse creates a task for merging and waits for execution on all nodes (if the replication_alter_partitions_sync setting is enabled).

  • If OPTIMIZE doesn’t perform a merge for any reason, it doesn’t notify the client. To enable notifications, use the optimize_throw_if_noop setting.
  • If you specify a PARTITION, only the specified partition is optimized. How to set partition expression.
  • If you specify FINAL, optimization is performed even when all the data is already in one part.
  • If you specify DEDUPLICATE, then completely identical rows will be deduplicated (all columns are compared), it makes sense only for the MergeTree engine.

!!! warning "Warning" OPTIMIZE can’t fix the “Too many parts” error.

RENAME

Renames one or more tables.

RENAME TABLE [db11.]name11 TO [db12.]name12, [db21.]name21 TO [db22.]name22, ... [ON CLUSTER cluster]

All tables are renamed under global locking. Renaming tables is a light operation. If you indicated another database after TO, the table will be moved to this database. However, the directories with databases must reside in the same file system (otherwise, an error is returned).

SET

SET param = value

Assigns value to the param setting for the current session. You cannot change server settings this way.

You can also set all the values from the specified settings profile in a single query.

SET profile = 'profile-name-from-the-settings-file'

For more information, see Settings.

TRUNCATE

TRUNCATE TABLE [IF EXISTS] [db.]name [ON CLUSTER cluster]

Removes all data from a table. When the clause IF EXISTS is omitted, the query returns an error if the table does not exist.

The TRUNCATE query is not supported for View, File, URL and Null table engines.

USE

USE db

Lets you set the current database for the session. The current database is used for searching for tables if the database is not explicitly defined in the query with a dot before the table name. This query can’t be made when using the HTTP protocol, since there is no concept of a session.

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