Manipulating Partitions and Parts 

The following operations with partitions are available:

DETACH PARTITION 

ALTER TABLE table_name DETACH PARTITION 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 visits DETACH PARTITION 201901

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

DROP PARTITION 

ALTER TABLE table_name DROP PARTITION 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 specify the partition expression.

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

DROP DETACHED PARTITION|PART 

ALTER TABLE table_name DROP DETACHED PARTITION|PART 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 specify the partition expression.

ATTACH PARTITION|PART 

ALTER TABLE table_name 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 specify 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. All other replicas download the data from the replica-initiator.

So 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 ATTACH PARTITION partition_expr FROM table1

This query copies the data partition from the table1 to table2 adds data to exsisting in the table2. Note that data won’t be deleted from table1.

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.

REPLACE PARTITION 

ALTER TABLE table2 REPLACE PARTITION partition_expr FROM table1

This query copies the data partition from the table1 to table2 and replaces existing partition in the table2. Note that data won’t be deleted from table1.

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.

MOVE PARTITION TO TABLE 

ALTER TABLE table_source 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.
  • Both tables must be the same engine family (replicated or non-replicated).
  • Both tables must have the same storage policy.

CLEAR COLUMN IN PARTITION 

ALTER TABLE table_name 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 FREEZE [PARTITION partition_expr]

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 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 specify 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.

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 doesn’t require stopping the server.

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

CLEAR INDEX IN PARTITION 

ALTER TABLE table_name 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 

ALTER TABLE table_name FETCH PARTITION 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 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:

ALTER TABLE users FETCH PARTITION 201902 FROM '/clickhouse/tables/01-01/visits';
ALTER TABLE users ATTACH PARTITION 201902;

Note that:

  • The ALTER ... FETCH PARTITION query isn’t replicated. It places the 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 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'

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.
  • As the expression from the table column. Constants and constant expressions are supported. For example, ALTER TABLE visits DETACH PARTITION 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;

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

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