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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 index-
       name. 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 ta-
       ble,  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  [ana-
       lyze(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         2014-02-17                        CLUSTER(7)