CLUSTER(7)							   SQL Commands 							CLUSTER(7)

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.  How-
       ever,  if you tend to access some data more than others, and there is an index that groups them together, you will benefit from using CLUS-
       TER.

       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 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. Other-
       wise, 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.

SQL - Language Statements					    2002-11-22								CLUSTER(7)