CLUSTER(7)                                                SQL Commands                                                CLUSTER(7)



NAME
       CLUSTER - cluster a table according to an index


SYNOPSIS
       CLUSTER [VERBOSE] tablename [ USING indexname ]
       CLUSTER [VERBOSE]


DESCRIPTION
       CLUSTER  instructs  PostgreSQL to cluster the table specified by tablename 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.
       Also,  setting  the  table's FILLFACTOR storage parameter to less than 100% can aid in preserving cluster ordering during
       updates, since updated rows are preferentially kept on the same page.)

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

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

       indexname
              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  unim-
       portant.  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.

       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 might make poor choices of query plans.

       There  is another way to cluster data. The CLUSTER command reorders the original table by scanning it using the index you
       specify. This can be slow on large tables because the rows are fetched from the table in index order, and if the table is
       disordered,  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 * FROM table ORDER BY columnlist;

       which uses the PostgreSQL sorting code to produce the desired order; this is usually much faster than an index  scan  for
       disordered  data.  Then you drop the old table, use ALTER TABLE ... RENAME to rename newtable to the old name, and recre-
       ate the table's indexes.  The big disadvantage of this approach is that it does not preserve OIDs,  constraints,  foreign
       key  relationships,  granted  privileges,  and other ancillary properties of the table -- all such items must be manually
       recreated. Another disadvantage is that this way requires a sort temporary file about the same size as the table  itself,
       so peak disk usage is about three times the table size instead of twice the table size.

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 indexname ON tablename

       is also supported for compatibility with pre-8.3 PostgreSQL versions.

SEE ALSO
       clusterdb [clusterdb(1)]



SQL - Language Statements                                  2011-09-22                                                 CLUSTER(7)