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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 of a table.

  OUTPUTS

  CLUSTER
       The clustering was done successfully.

  ERROR: relation <tablerelation_number> inherits "table"
       [Comment:  This is not documented anywhere. It seems not to be
       possible to cluster a table that is inherited.  ]

  ERROR: Relation table does not exist!
       [Comment:  The specified relation was not shown in the error message,
       which contained a random string instead of the relation name.  ]

DESCRIPTION
  CLUSTER instructs PostgreSQL to cluster the table specified by table
  approximately 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. The clustering is static.  In other words, as the table is
  updated, the changes are not clustered. No attempt is made to keep new
  instances or updated tuples clustered. If one wishes, one can re-cluster
  manually by issuing the command again.

  NOTES

  The table is actually copied to a temporary table in index order, then
  renamed back to the original name. For this reason, all grant permissions
  and other indexes are lost when clustering is performed.

  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.

  There are two ways to cluster data. The first is with the CLUSTER command,
  which reorders the original table with 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.

  Another way to cluster data 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 match the
  index, and which is much faster 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. The only problem is that OIDs will not be
  preserved. From then on, CLUSTER should be fast because most of the heap
  data has already been ordered, and the existing index is used.

USAGE
  Cluster the employees relation on the basis of its salary 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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