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Manipulating Partitions and Parts

The following operations with partitions are available:

DETACH PARTITION|PART​

ALTER TABLE table_name [ON CLUSTER cluster] DETACH PARTITION|PART partition_expr

Moves all data for the specified partition to the detached directory. The server forgets about the detached data partition as if it does not exist. The server will not know about this data until you make the ATTACH query.

Example:

ALTER TABLE mt DETACH PARTITION '2020-11-21';
ALTER TABLE mt DETACH PART 'all_2_2_0';

Read about setting the partition expression in a section How to set the partition expression.

After the query is executed, you can do whatever you want with the data in the detached directory — delete it from the file system, or just leave it.

This query is replicated – it moves the data to the detached directory on all replicas. Note that you can execute this query only on a leader replica. To find out if a replica is a leader, perform the SELECT query to the system.replicas table. Alternatively, it is easier to make a DETACH query on all replicas - all the replicas throw an exception, except the leader replicas (as multiple leaders are allowed).

DROP PARTITION|PART​

ALTER TABLE table_name [ON CLUSTER cluster] DROP PARTITION|PART partition_expr

Deletes the specified partition from the table. This query tags the partition as inactive and deletes data completely, approximately in 10 minutes.

Read about setting the partition expression in a section How to set the partition expression.

The query is replicated – it deletes data on all replicas.

Example:

ALTER TABLE mt DROP PARTITION '2020-11-21';
ALTER TABLE mt DROP PART 'all_4_4_0';

DROP DETACHED PARTITION|PART​

ALTER TABLE table_name [ON CLUSTER cluster] DROP DETACHED PARTITION|PART ALL|partition_expr

Removes the specified part or all parts of the specified partition from detached. Read more about setting the partition expression in a section How to set the partition expression.

FORGET PARTITION​

ALTER TABLE table_name FORGET PARTITION partition_expr

Removes all metadata about an empty partition from ZooKeeper. Query fails if partition is not empty or unknown. Make sure to execute only for partitions that will never be used again.

Read about setting the partition expression in a section How to set the partition expression.

Example:

ALTER TABLE mt FORGET PARTITION '20201121';

ATTACH PARTITION|PART​

ALTER TABLE table_name [ON CLUSTER cluster] ATTACH PARTITION|PART partition_expr

Adds data to the table from the detached directory. It is possible to add data for an entire partition or for a separate part. Examples:

ALTER TABLE visits ATTACH PARTITION 201901;
ALTER TABLE visits ATTACH PART 201901_2_2_0;

Read more about setting the partition expression in a section How to set the partition expression.

This query is replicated. The replica-initiator checks whether there is data in the detached directory. If data exists, the query checks its integrity. If everything is correct, the query adds the data to the table.

If the non-initiator replica, receiving the attach command, finds the part with the correct checksums in its own detached folder, it attaches the data without fetching it from other replicas. If there is no part with the correct checksums, the data is downloaded from any replica having the part.

You can put data to the detached directory on one replica and use the ALTER ... ATTACH query to add it to the table on all replicas.

ATTACH PARTITION FROM​

ALTER TABLE table2 [ON CLUSTER cluster] ATTACH PARTITION partition_expr FROM table1

This query copies the data partition from table1 to table2.

Note that:

  • Data will be deleted neither from table1 nor from table2.
  • table1 may be a temporary table.

For the query to run successfully, the following conditions must be met:

  • Both tables must have the same structure.
  • Both tables must have the same partition key, the same order by key and the same primary key.
  • Both tables must have the same indices and projections.
  • Both tables must have the same storage policy.

REPLACE PARTITION​

ALTER TABLE table2 [ON CLUSTER cluster] REPLACE PARTITION partition_expr FROM table1

This query copies the data partition from table1 to table2 and replaces the existing partition in table2. The operation is atomic.

Note that:

  • Data won’t be deleted from table1.
  • table1 may be a temporary table.

For the query to run successfully, the following conditions must be met:

  • Both tables must have the same structure.
  • Both tables must have the same partition key, the same order by key and the same primary key.
  • Both tables must have the same indices and projections.
  • Both tables must have the same storage policy.

MOVE PARTITION TO TABLE​

ALTER TABLE table_source [ON CLUSTER cluster] MOVE PARTITION partition_expr TO TABLE table_dest

This query moves the data partition from the table_source to table_dest with deleting the data from table_source.

For the query to run successfully, the following conditions must be met:

  • Both tables must have the same structure.
  • Both tables must have the same partition key, the same order by key and the same primary key.
  • Both tables must have the same indices and projections.
  • Both tables must have the same storage policy.
  • Both tables must be the same engine family (replicated or non-replicated).

CLEAR COLUMN IN PARTITION​

ALTER TABLE table_name [ON CLUSTER cluster] CLEAR COLUMN column_name IN PARTITION partition_expr

Resets all values in the specified column in a partition. If the DEFAULT clause was determined when creating a table, this query sets the column value to a specified default value.

Example:

ALTER TABLE visits CLEAR COLUMN hour in PARTITION 201902

FREEZE PARTITION​

ALTER TABLE table_name [ON CLUSTER cluster] FREEZE [PARTITION partition_expr] [WITH NAME 'backup_name']

This query creates a local backup of a specified partition. If the PARTITION clause is omitted, the query creates the backup of all partitions at once.

Note

The entire backup process is performed without stopping the server.

Note that for old-styled tables you can specify the prefix of the partition name (for example, 2019) - then the query creates the backup for all the corresponding partitions. Read about setting the partition expression in a section How to set the partition expression.

At the time of execution, for a data snapshot, the query creates hardlinks to a table data. Hardlinks are placed in the directory /var/lib/clickhouse/shadow/N/..., where:

  • /var/lib/clickhouse/ is the working ClickHouse directory specified in the config.
  • N is the incremental number of the backup.
  • if the WITH NAME parameter is specified, then the value of the 'backup_name' parameter is used instead of the incremental number.
Note

If you use a set of disks for data storage in a table, the shadow/N directory appears on every disk, storing data parts that matched by the PARTITION expression.

The same structure of directories is created inside the backup as inside /var/lib/clickhouse/. The query performs chmod for all files, forbidding writing into them.

After creating the backup, you can copy the data from /var/lib/clickhouse/shadow/ to the remote server and then delete it from the local server. Note that the ALTER t FREEZE PARTITION query is not replicated. It creates a local backup only on the local server.

The query creates backup almost instantly (but first it waits for the current queries to the corresponding table to finish running).

ALTER TABLE t FREEZE PARTITION copies only the data, not table metadata. To make a backup of table metadata, copy the file /var/lib/clickhouse/metadata/database/table.sql

To restore data from a backup, do the following:

  1. Create the table if it does not exist. To view the query, use the .sql file (replace ATTACH in it with CREATE).
  2. Copy the data from the data/database/table/ directory inside the backup to the /var/lib/clickhouse/data/database/table/detached/ directory.
  3. Run ALTER TABLE t ATTACH PARTITION queries to add the data to a table.

Restoring from a backup does not require stopping the server.

For more information about backups and restoring data, see the Data Backup section.

UNFREEZE PARTITION​

ALTER TABLE table_name [ON CLUSTER cluster] UNFREEZE [PARTITION 'part_expr'] WITH NAME 'backup_name'

Removes freezed partitions with the specified name from the disk. If the PARTITION clause is omitted, the query removes the backup of all partitions at once.

CLEAR INDEX IN PARTITION​

ALTER TABLE table_name [ON CLUSTER cluster] CLEAR INDEX index_name IN PARTITION partition_expr

The query works similar to CLEAR COLUMN, but it resets an index instead of a column data.

FETCH PARTITION|PART​

ALTER TABLE table_name [ON CLUSTER cluster] FETCH PARTITION|PART partition_expr FROM 'path-in-zookeeper'

Downloads a partition from another server. This query only works for the replicated tables.

The query does the following:

  1. Downloads the partition|part from the specified shard. In ‘path-in-zookeeper’ you must specify a path to the shard in ZooKeeper.
  2. Then the query puts the downloaded data to the detached directory of the table_name table. Use the ATTACH PARTITION|PART query to add the data to the table.

For example:

  1. FETCH PARTITION
ALTER TABLE users FETCH PARTITION 201902 FROM '/clickhouse/tables/01-01/visits';
ALTER TABLE users ATTACH PARTITION 201902;
  1. FETCH PART
ALTER TABLE users FETCH PART 201901_2_2_0 FROM '/clickhouse/tables/01-01/visits';
ALTER TABLE users ATTACH PART 201901_2_2_0;

Note that:

  • The ALTER ... FETCH PARTITION|PART query isn’t replicated. It places the part or partition to the detached directory only on the local server.
  • The ALTER TABLE ... ATTACH query is replicated. It adds the data to all replicas. The data is added to one of the replicas from the detached directory, and to the others - from neighboring replicas.

Before downloading, the system checks if the partition exists and the table structure matches. The most appropriate replica is selected automatically from the healthy replicas.

Although the query is called ALTER TABLE, it does not change the table structure and does not immediately change the data available in the table.

MOVE PARTITION|PART​

Moves partitions or data parts to another volume or disk for MergeTree-engine tables. See Using Multiple Block Devices for Data Storage.

ALTER TABLE table_name [ON CLUSTER cluster] MOVE PARTITION|PART partition_expr TO DISK|VOLUME 'disk_name'

The ALTER TABLE t MOVE query:

  • Not replicated, because different replicas can have different storage policies.
  • Returns an error if the specified disk or volume is not configured. Query also returns an error if conditions of data moving, that specified in the storage policy, can’t be applied.
  • Can return an error in the case, when data to be moved is already moved by a background process, concurrent ALTER TABLE t MOVE query or as a result of background data merging. A user shouldn’t perform any additional actions in this case.

Example:

ALTER TABLE hits MOVE PART '20190301_14343_16206_438' TO VOLUME 'slow'
ALTER TABLE hits MOVE PARTITION '2019-09-01' TO DISK 'fast_ssd'

UPDATE IN PARTITION​

Manipulates data in the specifies partition matching the specified filtering expression. Implemented as a mutation.

Syntax:

ALTER TABLE [db.]table [ON CLUSTER cluster] UPDATE column1 = expr1 [, ...] [IN PARTITION partition_expr] WHERE filter_expr

Example​

-- using partition name
ALTER TABLE mt UPDATE x = x + 1 IN PARTITION 2 WHERE p = 2;

-- using partition id
ALTER TABLE mt UPDATE x = x + 1 IN PARTITION ID '2' WHERE p = 2;

See Also​

DELETE IN PARTITION​

Deletes data in the specifies partition matching the specified filtering expression. Implemented as a mutation.

Syntax:

ALTER TABLE [db.]table [ON CLUSTER cluster] DELETE [IN PARTITION partition_expr] WHERE filter_expr

Example​

-- using partition name
ALTER TABLE mt DELETE IN PARTITION 2 WHERE p = 2;

-- using partition id
ALTER TABLE mt DELETE IN PARTITION ID '2' WHERE p = 2;

See Also​

How to Set Partition Expression​

You can specify the partition expression in ALTER ... PARTITION queries in different ways:

  • As a value from the partition column of the system.parts table. For example, ALTER TABLE visits DETACH PARTITION 201901.
  • Using the keyword ALL. It can be used only with DROP/DETACH/ATTACH/ATTACH FROM. For example, ALTER TABLE visits ATTACH PARTITION ALL.
  • As a tuple of expressions or constants that matches (in types) the table partitioning keys tuple. In the case of a single element partitioning key, the expression should be wrapped in the tuple (...) function. For example, ALTER TABLE visits DETACH PARTITION tuple(toYYYYMM(toDate('2019-01-25'))).
  • Using the partition ID. Partition ID is a string identifier of the partition (human-readable, if possible) that is used as the names of partitions in the file system and in ZooKeeper. The partition ID must be specified in the PARTITION ID clause, in a single quotes. For example, ALTER TABLE visits DETACH PARTITION ID '201901'.
  • In the ALTER ATTACH PART and DROP DETACHED PART query, to specify the name of a part, use string literal with a value from the name column of the system.detached_parts table. For example, ALTER TABLE visits ATTACH PART '201901_1_1_0'.

Usage of quotes when specifying the partition depends on the type of partition expression. For example, for the String type, you have to specify its name in quotes ('). For the Date and Int* types no quotes are needed.

All the rules above are also true for the OPTIMIZE query. If you need to specify the only partition when optimizing a non-partitioned table, set the expression PARTITION tuple(). For example:

OPTIMIZE TABLE table_not_partitioned PARTITION tuple() FINAL;

IN PARTITION specifies the partition to which the UPDATE or DELETE expressions are applied as a result of the ALTER TABLE query. New parts are created only from the specified partition. In this way, IN PARTITION helps to reduce the load when the table is divided into many partitions, and you only need to update the data point-by-point.

The examples of ALTER ... PARTITION queries are demonstrated in the tests 00502_custom_partitioning_local and 00502_custom_partitioning_replicated_zookeeper.