Provided by: postgresql-client-8.4_8.4.8-2_i386 bug

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 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)]