CLUSTER()			 SQL Commands			     CLUSTER()



NAME
       CLUSTER - cluster a table according to an index


SYNOPSIS
       CLUSTER indexname ON tablename
       CLUSTER tablename
       CLUSTER


DESCRIPTION
       CLUSTER	instructs  PostgreSQL to cluster the table specified by table-
       name based on the index specified by indexname. The index must  already
       have been defined on tablename.

       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.

       When a table is clustered, PostgreSQL remembers on which index  it  was
       clustered.  The form CLUSTER tablename reclusters the table on the same
       index that it was clustered before.

       CLUSTER without any parameter reclusters all the tables in the  current
       database that the calling user owns, or all tables if called by a supe-
       ruser. (Never-clustered tables are not included.) This form of  CLUSTER
       cannot be called from inside a transaction or function.

       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
       indexname
	      The name of an index.

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

NOTES
       CLUSTER loses all visibility information of tuples, which makes the ta-
       ble look empty to any snapshot that was taken before the	 CLUSTER  com-
       mand  finished.	That  makes  CLUSTER unsuitable for applications where
       transactions that access the table being clustered are run concurrently
       with  CLUSTER.  This  is	 most  visible with serializable transactions,
       because they take only one snapshot at the beginning  of	 the  transac-
       tion, but read-committed transactions are also affected.

       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 heap page for the first row that
       matches, all other rows that match are probably	already	 on  the  same
       heap page, and so you save disk accesses and speed up the query.

       During  the cluster operation, 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.

       Because	CLUSTER	 remembers the clustering information, one can cluster
       the tables one wants clustered manually the first  time,	 and  setup  a
       timed  event  similar  to  VACUUM  so  that the tables are periodically
       reclustered.

       Because the planner records statistics about the ordering of tables, it
       is  advisable to run ANALYZE [analyze(7)] on the newly clustered table.
       Otherwise, the planner may make poor choices of query plans.

       There is another way to cluster data. The CLUSTER command reorders  the
       original table using the ordering of the index you specify. This can be
       slow on large tables because the rows are  fetched  from	 the  heap  in
       index  order,  and  if  the heap table is unordered, the entries are on
       random pages, so there is one disk page retrieved for every row	moved.
       (PostgreSQL  has	 a cache, but the majority of a big table will not fit
       in the cache.)  The other way to cluster a table is to use

       CREATE TABLE newtable AS
	   SELECT columnlist FROM table ORDER BY columnlist;

       which uses the PostgreSQL sorting code in the ORDER BY clause to create
       the  desired  order; this is usually much faster than an index scan for
       unordered data. You then drop the old table, use ALTER TABLE ... RENAME
       to  rename  newtable to the old name, and recreate the table's indexes.
       However, this approach does not preserve OIDs, constraints, foreign key
       relationships,  granted	privileges,  and other ancillary properties of
       the table -- all such items must be manually recreated.

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

       CLUSTER emp_ind ON emp;


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

       CLUSTER emp;


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

       CLUSTER;


COMPATIBILITY
       There is no CLUSTER statement in the SQL standard.

SEE ALSO
       clusterdb [clusterdb(1)]



SQL - Language Statements	  2010-12-14			     CLUSTER()