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CLUSTER(7)                         PostgreSQL 14.23 Documentation                         CLUSTER(7)



NAME
       CLUSTER - cluster a table according to an index

SYNOPSIS
       CLUSTER [VERBOSE] table_name [ USING index_name ]
       CLUSTER ( option [, ...] ) table_name [ USING index_name ]
       CLUSTER [VERBOSE]

       where option can be one of:

           VERBOSE [ boolean ]

DESCRIPTION
       CLUSTER instructs PostgreSQL to cluster the table specified by table_name based on the index
       specified by index_name. The index must already have been defined on table_name.

       When a table is clustered, it is physically reordered based on the index information.
       Clustering is a one-time operation: when the table is subsequently updated, the changes are
       not clustered. That is, no attempt is made to store new or updated rows according to their
       index order. (If one wishes, one can periodically recluster by issuing the command again.
       Also, setting the table's fillfactor storage parameter to less than 100% can aid in
       preserving cluster ordering during updates, since updated rows are kept on the same page if
       enough space is available there.)

       When a table is clustered, PostgreSQL remembers which index it was clustered by. The form
       CLUSTER table_name reclusters the table using the same index as before. You can also use the
       CLUSTER or SET WITHOUT CLUSTER forms of ALTER TABLE to set the index to be used for future
       cluster operations, or to clear any previous setting.

       CLUSTER without any parameter reclusters all the previously-clustered tables in the current
       database that the calling user owns, or all such tables if called by a superuser. This form
       of CLUSTER cannot be executed inside a transaction block.

       When a table is being clustered, an ACCESS EXCLUSIVE lock is acquired on it. This prevents
       any other database operations (both reads and writes) from operating on the table until the
       CLUSTER is finished.

PARAMETERS
       table_name
           The name (possibly schema-qualified) of a table.

       index_name
           The name of an index.

       VERBOSE
           Prints a progress report as each table is clustered.

       boolean
           Specifies whether the selected option should be turned on or off. You can write TRUE, ON,
           or 1 to enable the option, and FALSE, OFF, or 0 to disable it. The boolean value can also
           be omitted, in which case TRUE is assumed.

NOTES
       In cases where you are accessing single rows randomly within a table, the actual order of the
       data in the table is unimportant. However, if you tend to access some data more than others,
       and there is an index that groups them together, you will benefit from using CLUSTER. If you
       are requesting a range of indexed values from a table, or a single indexed value that has
       multiple rows that match, CLUSTER will help because once the index identifies the table page
       for the first row that matches, all other rows that match are probably already on the same
       table page, and so you save disk accesses and speed up the query.

       CLUSTER can re-sort the table using either an index scan on the specified index, or (if the
       index is a b-tree) a sequential scan followed by sorting. It will attempt to choose the
       method that will be faster, based on planner cost parameters and available statistical
       information.

       When an index scan is used, a temporary copy of the table is created that contains the table
       data in the index order. Temporary copies of each index on the table are created as well.
       Therefore, you need free space on disk at least equal to the sum of the table size and the
       index sizes.

       When a sequential scan and sort is used, a temporary sort file is also created, so that the
       peak temporary space requirement is as much as double the table size, plus the index sizes.
       This method is often faster than the index scan method, but if the disk space requirement is
       intolerable, you can disable this choice by temporarily setting enable_sort to off.

       It is advisable to set maintenance_work_mem to a reasonably large value (but not more than
       the amount of RAM you can dedicate to the CLUSTER operation) before clustering.

       Because the planner records statistics about the ordering of tables, it is advisable to run
       ANALYZE on the newly clustered table. Otherwise, the planner might make poor choices of query
       plans.

       Because CLUSTER remembers which indexes are clustered, one can cluster the tables one wants
       clustered manually the first time, then set up a periodic maintenance script that executes
       CLUSTER without any parameters, so that the desired tables are periodically reclustered.

       Each backend running CLUSTER will report its progress in the pg_stat_progress_cluster view.
       See Section 28.4.4 for details.

EXAMPLES
       Cluster the table employees on the basis of its index employees_ind:

           CLUSTER employees USING employees_ind;

       Cluster the employees table using the same index that was used before:

           CLUSTER employees;

       Cluster all tables in the database that have previously been clustered:

           CLUSTER;

COMPATIBILITY
       There is no CLUSTER statement in the SQL standard.

       The syntax

           CLUSTER index_name ON table_name

       is also supported for compatibility with pre-8.3 PostgreSQL versions.

SEE ALSO
       clusterdb(1), Section 28.4.4



PostgreSQL 14.23                                2026                                      CLUSTER(7)