ClickHouse/docs/en/query_language/misc.md

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 is only supported for the following table engines:

• Log
• TinyLog
• StripeLog

These engines do not provide automatic data recovery on failure. Use the CHECK TABLE query to track data loss in a timely manner.

To avoid data loss use the MergeTree family tables.

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 two String-type columns: name and type, which indicate the names and types of columns in the specified table.

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.

EXISTS

EXISTS [TEMPORARY] TABLE [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.

Original article

OPTIMIZE

OPTIMIZE TABLE [db.]name [ON CLUSTER cluster] [PARTITION partition] [FINAL]

Asks the table engine to do something for optimization. Supported only by *MergeTree engines, in which this query initializes a non-scheduled merge of data parts. If you specify a PARTITION, only the specified partition will be optimized. If you specify FINAL, optimization will be performed even when all the data is already in one part.

!!! 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

Allows you to set param to value. You can also make all the settings from the specified settings profile in a single query. To do this, specify 'profile' as the setting name. For more information, see the section "Settings". The setting is made for the session, or for the server (globally) if GLOBAL is specified. When making a global setting, the setting is not applied to sessions already running, including the current session. It will only be used for new sessions.

When the server is restarted, global settings made using SET are lost. To make settings that persist after a server restart, you can only use the server's config file.

SHOW CREATE TABLE

SHOW CREATE [TEMPORARY] TABLE [db.]table [INTO OUTFILE filename] [FORMAT format]

Returns a single String-type 'statement' column, which contains a single value – the CREATE query used for creating the specified table.

SHOW DATABASES

SHOW DATABASES [INTO OUTFILE filename] [FORMAT format]

Prints a list of all databases. This query is identical to SELECT name FROM system.databases [INTO OUTFILE filename] [FORMAT format].

See also the section "Formats".

SHOW PROCESSLIST

SHOW PROCESSLIST [INTO OUTFILE filename] [FORMAT format]

Outputs a list of queries currently being processed, other than SHOW PROCESSLIST queries.

Prints a table containing the columns:

user – The user who made the query. Keep in mind that for distributed processing, queries are sent to remote servers under the 'default' user. SHOW PROCESSLIST shows the username for a specific query, not for a query that this query initiated.

address – The name of the host that the query was sent from. For distributed processing, on remote servers, this is the name of the query requestor host. To track where a distributed query was originally made from, look at SHOW PROCESSLIST on the query requestor server.

elapsed – The execution time, in seconds. Queries are output in order of decreasing execution time.

rows_read, bytes_read – How many rows and bytes of uncompressed data were read when processing the query. For distributed processing, data is totaled from all the remote servers. This is the data used for restrictions and quotas.

memory_usage – Current RAM usage in bytes. See the setting 'max_memory_usage'.

query – The query itself. In INSERT queries, the data for insertion is not output.

query_id – The query identifier. Non-empty only if it was explicitly defined by the user. For distributed processing, the query ID is not passed to remote servers.

This query is identical to: SELECT * FROM system.processes [INTO OUTFILE filename] [FORMAT format].

Tip (execute in the console):

watch -n1 "clickhouse-client --query='SHOW PROCESSLIST'"

SHOW TABLES

SHOW [TEMPORARY] TABLES [FROM db] [LIKE 'pattern'] [INTO OUTFILE filename] [FORMAT format]

Displays a list of tables

• Tables from the current database, or from the 'db' database if "FROM db" is specified.
• All tables, or tables whose name matches the pattern, if "LIKE 'pattern'" is specified.

This query is identical to: SELECT name FROM system.tables WHERE database = 'db' [AND name LIKE 'pattern'] [INTO OUTFILE filename] [FORMAT format].

See also the section "LIKE operator".

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.