VACUUM TABLE

Introduced or updated: v1.2.39

ENTERPRISE EDITION FEATURE

VACUUM TABLE is an Enterprise Edition feature. Contact Databend Support for a license.

The VACUUM TABLE command helps optimize system performance by freeing up storage space through the permanent removal of historical data files from a table. This includes:

• Snapshots associated with the table, as well as their relevant segments and blocks.

• Orphan files. Orphan files in Databend refer to snapshots, segments, and blocks that are no longer associated with the table. Orphan files might be generated from various operations and errors, such as during data backups and restores, and can take up valuable disk space and degrade the system performance over time.

See also: VACUUM DROP TABLE

Syntax and Examples

VACUUM TABLE <table_name> [RETAIN n HOURS] [DRY RUN]

• RETAIN n HOURS: This option determines the retention period for historical data files. When this option is specified, Databend will compare the values of n and the setting retention_period, and it will use the larger value as the retention period. For example, if the specified value of n is greater than the default retention_period, then Databend will retain data files for n hours, so historical data files that are older than n hours will be removed.

  When this option is not specified, the default retention_period setting of 12 hours will be applied. This means that any historical data files that are older than 12 hours will be removed.

• DRY RUN: When this option is specified, candidate orphan files will not be removed, instead, a list of up to 1,000 candidate files will be returned that would have been removed if the option was not used. This is useful when you want to preview the potential impact of the VACUUM TABLE command on the table before actually removing any data files. For example:

  VACUUM TABLE t RETAIN 0 HOURS DRY RUN;
  
  +-----------------------------------------------------+
  | Files                                               |
  +-----------------------------------------------------+
  | 1/8/_sg/932addea38c64393b82cb4b8fb7a2177_v3.bincode |
  | 1/8/_b/b68cbe5fe015474d85a92d5f7d1b5d99_v2.parquet  |
  +-----------------------------------------------------+

VACUUM TABLE vs. OPTIMIZE TABLE

Databend provides two commands for removing historical data files from a table: VACUUM TABLE and OPTIMIZE TABLE (with the PURGE option). Although both commands are capable of permanently deleting data files, they differ in how they handle orphan files: OPTIMIZE TABLE is able to remove orphan snapshots, as well as the corresponding segments and blocks. However, there is a possibility of orphan segments and blocks existing without any associated snapshots. In such a scenario, only VACUUM TABLE can help clean them up.

Both VACUUM TABLE and OPTIMIZE TABLE allow you to specify a period to determine which historical data files to remove. However, OPTIMIZE TABLE requires you to obtain the snapshot ID or timestamp from a query beforehand, whereas VACUUM TABLE allows you to specify the number of hours to retain the data files directly. VACUUM TABLE provides enhanced control over your historical data files both before their removal with the DRY RUN option, which allows you to preview the data files to be removed before applying the command. This provides a safe removal experience and helps you avoid unintended data loss.

	VACUUM TABLE	OPTIMIZE TABLE
Associated snapshots (incl. segments and blocks)	Yes	Yes
Orphan snapshots (incl. segments and blocks)	Yes	Yes
Orphan segments and blocks only	Yes	No
DRY RUN	Yes	No