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CLUSTER(7)		PostgreSQL 9.6.3 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 9.6.3		     2017			    CLUSTER(7)

NAME | SYNOPSIS | DESCRIPTION | PARAMETERS | NOTES | EXAMPLES | COMPATIBILITY | SEE ALSO

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