CLUSTER(7)						  PostgreSQL 9.2.7 Documentation						CLUSTER(7)

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)

PostgreSQL 9.2.7						    2014-02-17								CLUSTER(7)