CLUSTER(l) SQL - Language Statements (2002-11-22) CLUSTER(l)
NAME
CLUSTER - cluster a table according to an index
SYNOPSIS
CLUSTER indexname ON tablename
INPUTS
indexname
The name of an index.
table
The name (possibly schema-qualified) of a table.
OUTPUTS
CLUSTER
The clustering was done successfully.
DESCRIPTION
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 again.
NOTES
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 CLUSTER.
Another place where CLUSTER is helpful is in cases where you
use an index to pull out several 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 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
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CLUSTER(l) SQL - Language Statements (2002-11-22) CLUSTER(l)
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 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.
USAGE
Cluster the employees relation on the basis of its ID
attribute:
CLUSTER emp_ind ON emp;
COMPATIBILITY
SQL92
There is no CLUSTER statement in SQL92.
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