NAME

       CLUSTER - cluster a table according to an index

SYNOPSIS

       CLUSTER [VERBOSE] table_name [ USING index_name ]
       CLUSTER [VERBOSE]

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 (ALTER_TABLE(7)) 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.

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(7) 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.

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)