CLUSTER instructs PostgreSQL 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.
The table is actually copied to a temporary table in index order, then renamed back to the original name. For this reason, all grant permissions 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 command, 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. PostgreSQL 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 PostgreSQL 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 newtable to the old name, and recreate the table's 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.