CLUSTER indexname ON table
Use the CLUSTER command to cluster a table with a specific index. The table (identified by the table parameter) must already exist and the index (identified by the index parameter) must exist on the table.
During clustering, a table is reordered in an arrangement based on the supplied index. A cluster is a one-time operation upon a table; to re-cluster a table, you must execute the command again.
When a cluster is created, the table is actually being copied to a temporary table according to index order, then it is renamed to the original table name. Because of this, all permissions and other indexes are lost when clustering is performed.