CLUSTER(7) SQL Commands CLUSTER(7)
CLUSTER - cluster a table according to an index
CLUSTER indexname ON tablename
The name of an index.
table The name (possibly schema-qualified) of a table.
The clustering was done successfully.
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
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
Another place where CLUSTER is helpful is in cases where you use an index to pull out sev-
eral 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
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 ta-
ble'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.
Cluster the employees relation on the basis of its ID attribute:
CLUSTER emp_ind ON emp;
There is no CLUSTER statement in SQL92.
SQL - Language Statements 2002-11-22 CLUSTER(7)