Provided by: postgresql-client-8.3_8.3.7-1_i386 bug

NAME

       CLUSTER - cluster a table according to an index

SYNOPSIS

       CLUSTER tablename [ USING indexname ]
       CLUSTER

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.

NOTES

       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 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 recreate 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)]