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     CLUSTER(l)	  SQL - Language Statements (2002-11-22)    CLUSTER(l)

     NAME
	  CLUSTER - cluster a table according to an index

     SYNOPSIS
	  CLUSTER indexname ON tablename

	INPUTS
	  indexname
	       The name of an index.

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

	OUTPUTS
	  CLUSTER
	       The clustering was done successfully.

     DESCRIPTION
	  CLUSTER instructs PostgreSQL to cluster the table specified
	  by table 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 tuples according to their index order.
	  If one wishes, one can periodically re-cluster by issuing
	  the command again.

	NOTES
	  In cases where you are accessing single rows randomly within
	  a table, the actual order of the data in the heap 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.

	  Another place where CLUSTER is helpful is in cases where you
	  use an index to pull out several rows from a table. 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, saving
	  disk accesses and speeding 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

     Page 1					     (printed 3/24/03)

     CLUSTER(l)	  SQL - Language Statements (2002-11-22)    CLUSTER(l)

	  to the sum of the table size and the index sizes.

	  CLUSTER preserves GRANT, inheritance, index, foreign key,
	  and other ancillary information about the table.

	  Because the optimizer records statistics about the ordering
	  of tables, it is advisable to run ANALYZE on the newly
	  clustered table. Otherwise, the optimizer 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

	  SELECT columnlist INTO TABLE newtable
	       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.

     USAGE
	  Cluster the employees relation on the basis of its ID
	  attribute:

	  CLUSTER emp_ind ON emp;

     COMPATIBILITY
	SQL92
	  There is no CLUSTER statement in SQL92.

     Page 2					     (printed 3/24/03)

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