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.
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 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.