cluster

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
       superuser. (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
       table look empty to any snapshot that was  taken	 before	 the  CLUSTER
       command finished. That makes CLUSTER unsuitable for applications where
       transactions that access the table being	 clustered  are	 run  concur-
       rently  with  CLUSTER. This is most visible with serializable transac-
       tions, because they take only one snapshot at  the  beginning  of  the
       transaction, 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 on the newly clustered table. Other-
       wise, 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	 cre-
       ate  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	relation  using	 the same index that was used
       before:

       CLUSTER emp;


       Cluster all the tables on the database that have previously been clus-
       tered:

       CLUSTER;


COMPATIBILITY
       There is no CLUSTER statement in the SQL standard.

SEE ALSO
       clusterdb [clusterdb(1)]



SQL - Language Statements	  2008-01-03			    CLUSTER()