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CLUSTER(5)
NAME
  CLUSTER - cluster a table according to an index

SYNOPSIS
  CLUSTER indexname ON tablename

  INPUTS

  indexname
       The name of an index.

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

  OUTPUTS

  CLUSTER
       The clustering was done successfully.

DESCRIPTION
  CLUSTER instructs PostgreSQL to cluster the table specified by table 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 tuples according to their index order. If one
  wishes, one can periodically re-cluster by issuing the command again.

  NOTES

  In cases where you are accessing single rows randomly within a table, the
  actual order of the data in the heap 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.

  Another place where CLUSTER is helpful is in cases where you use an index
  to pull out several rows from a table. 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
  heap page for the first row that matches, all other rows that match are
  probably already on the same heap page, saving disk accesses and speeding
  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.

  CLUSTER preserves GRANT, inheritance, index, foreign key, and other
  ancillary information about the table.

  Because the optimizer records statistics about the ordering of tables, it
  is advisable to run ANALYZE on the newly clustered table. Otherwise, the
  optimizer may make poor choices of query plans.

  There is another way to cluster data. The CLUSTER command reorders the
  original table using the ordering of the index you specify. This can be
  slow on large tables because the rows are fetched from the heap in index
  order, and if the heap table is unordered, 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

  SELECT columnlist INTO TABLE newtable
       FROM table ORDER BY columnlist

  which uses the PostgreSQL sorting code in the ORDER BY clause to create the
  desired order; this is usually much faster than an index scan for unordered
  data. You then drop the old table, use ALTER TABLE...RENAME to rename
  newtable to the old name, and recreate the table's indexes. However, this
  approach does not preserve OIDs, constraints, foreign key relationships,
  granted privileges, and other ancillary properties of the table --- all
  such items must be manually recreated.

USAGE
  Cluster the employees relation on the basis of its ID attribute:

  CLUSTER emp_ind ON emp;

COMPATIBILITY
  SQL92

  There is no CLUSTER statement in SQL92.

Index for Section 5

Alphabetical listing for C

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