Provided by: postgresql-client-8.2_8.2.7-1_i386 bug

NAME

       CLUSTER - cluster a table according to an index

SYNOPSIS

       CLUSTER indexname ON tablename
       CLUSTER tablename
       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.

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

       CLUSTER without any parameter reclusters all the tables in the  current
       database  that  the  calling  user  owns,  or all tables if called by a
       superuser. (Never-clustered tables are  not  included.)  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

       indexname
              The name of an index.

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

NOTES

       CLUSTER  loses  all  visibility  information of tuples, which makes the
       table look empty to any snapshot that  was  taken  before  the  CLUSTER
       command  finished. That makes CLUSTER unsuitable for applications where
       transactions that access the table being clustered are run concurrently
       with  CLUSTER.  This  is  most  visible with serializable transactions,
       because  they  take  only  one  snapshot  at  the  beginning   of   the
       transaction, but read-committed transactions are also affected.

       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 may 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 emp_ind:

       CLUSTER emp_ind ON emp;

       Cluster the employees table using the same index that was used before:

       CLUSTER emp;

       Cluster all tables in the database that have previously been clustered:

       CLUSTER;

COMPATIBILITY

       There is no CLUSTER statement in the SQL standard.

SEE ALSO

       clusterdb [clusterdb(1)]