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CLUSTER(7)                       SQL Commands                       CLUSTER(7)

NAME
       CLUSTER - cluster a table according to an index

SYNOPSIS
       CLUSTER [VERBOSE] tablename [ USING indexname ]
       CLUSTER [VERBOSE]

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. Also, setting the table's FILLFACTOR storage parameter
       to less than 100% can aid in preserving cluster ordering during
       updates, since updated rows are preferentially kept on the same page.)

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

       CLUSTER without any parameter reclusters all the previously-clustered
       tables in the current database that the calling user owns, or all such
       tables if called by a superuser. 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
       tablename
              The name (possibly schema-qualified) of a table.

       indexname
              The name of an index.

       VERBOSE
              Prints a progress report as each table is clustered.

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 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 might 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 employees_ind:

       CLUSTER employees USING employees_ind;

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

       CLUSTER employees;

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

       CLUSTER;

COMPATIBILITY
       There is no CLUSTER statement in the SQL standard.

       The syntax

       CLUSTER indexname ON tablename

       is also supported for compatibility with pre-8.3 PostgreSQL versions.

SEE ALSO
       clusterdb [clusterdb(1)]

SQL - Language Statements         2014-07-21                        CLUSTER(7)