CLUSTER() SQL Commands CLUSTER()NAME
CLUSTER - Gives storage clustering advice to the server
SYNOPSIS
CLUSTER indexname ON tablename
INPUTS
indexname
The name of an index.
table The name of a table.
OUTPUTS
CLUSTER
The clustering was done successfully.
ERROR: relation <tablerelation_number> inherits "table"
[Comment: This is not documented anywhere. It seems not to be
possible to cluster a table that is inherited. ]
ERROR: Relation table does not exist!
[Comment: The specified relation was not shown in the error
message, which contained a random string instead of the relation
name. ]
DESCRIPTION
CLUSTER instructs Postgres to cluster the table specified by table
approximately 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. The clustering is static. In other words, as the
table is updated, the changes are not clustered. No attempt is made to
keep new instances or updated tuples clustered. If one wishes, one can
re-cluster manually by issuing the command again.
NOTES
The table is actually copied to a temporary table in index order, then
renamed back to the original name. For this reason, all grant permis‐
sions and other indexes are lost when clustering is performed.
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.
There are two ways to cluster data. The first is with the CLUSTER com‐
mand, which reorders the original table with 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. Postgres has a cache, but the majority
of a big table will not fit in the cache.
Another way to cluster data is to use
SELECT columnlist INTO TABLE newtable
FROM table ORDER BY columnlist
which uses the Postgres sorting code in the ORDER BY clause to match
the index, and which is much faster for unordered data. You then drop
the old table, use ALTER TABLE/RENAME to rename temp to the old name,
and recreate any indexes. The only problem is that OIDs will not be
preserved. From then on, CLUSTER should be fast because most of the
heap data has already been ordered, and the existing index is used.
USAGE
Cluster the employees relation on the basis of its salary attribute:
CLUSTER emp_ind ON emp;
COMPATIBILITY
SQL92
There is no CLUSTER statement in SQL92.
SQL - Language Statements 29 March 2001 CLUSTER()