CLUSTER() SQL Commands CLUSTER()NAME
CLUSTER - cluster a table according to an index
CLUSTER indexname ON tablename
CLUSTER instructs PostgreSQL to cluster the table specified by table‐
name 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
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 supe‐
ruser. (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.
The name of an index.
The name (possibly schema-qualified) of a table.
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
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
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 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.
Cluster the table employees on the basis of its index emp_ind:
CLUSTER emp_ind ON emp;
Cluster the employees table using the same index that was used before:
Cluster all tables in the database that have previously been clustered:
There is no CLUSTER statement in the SQL standard.
SQL - Language Statements 2005-11-05 CLUSTER()