cluster(5)

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 table-
       name 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 supe-
       ruser. (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 ta-
       ble look empty to any snapshot that was taken before the  CLUSTER  com-
       mand  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  transac-
       tion, 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 ta-
       ble 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(5)] 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.  (Post-
       greSQL 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,  con-
       straints,  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 tempo-
       rary 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)]

SQL - Language Statements         2008-06-08                         CLUSTER()