Index for Section 5

Alphabetical listing for C

Bottom of page

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 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 called from inside a transaction or function.

  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
  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 heap page for the first row that matches, all other rows
  that match are probably already on the same heap 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 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 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

  CREATE TABLE newtable AS
      SELECT columnlist 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.

EXAMPLES
  Cluster the table employees on the basis of its index emp_ind:

  CLUSTER emp_ind ON emp;

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

  CLUSTER emp;

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

  CLUSTER;

COMPATIBILITY
  There is no CLUSTER statement in the SQL standard.

SEE ALSO
  clusterdb [clusterdb(1)]

Index for Section 5

Alphabetical listing for C

Top of page