CLUSTER instructs PostgreSQL to cluster the table specified by tablename 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 rows according to their index order. If one wishes, one can periodically recluster by issuing the command again.
When a table is clustered, PostgreSQL remembers on which index it was clustered. The form CLUSTER tablename, reclusters the table on the same index that it was clustered before.
CLUSTER without any parameter reclusters all the tables in the current database that the calling user owns, or all tables if called by a superuser. (Never-clustered tables are not included.) This form of CLUSTER cannot be called from inside a transaction or function.
When a table is being clustered, an ACCESS EXCLUSIVE lock is acquired on it. This prevents any other database operations (both reads and writes) from operating on the table until the CLUSTER is finished.
The name of an index.
The name (possibly schema-qualified) of a table.
CLUSTER loses all visibility information of tuples, which makes the table look empty to any snapshot that was taken before the CLUSTER command finished. That makes CLUSTER unsuitable for applications where transactions that access the table being clustered are run concurrently with CLUSTER. This is most visible with serializable transactions, because they take only one snapshot at the beginning of the transaction, but read-committed transactions are also affected.
In cases where you are accessing single rows randomly within a table, the actual order of the data in the 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. 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, and so you save disk accesses and speed 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.
Because CLUSTER remembers the clustering information, one can cluster the tables one wants clustered manually the first time, and setup a timed event similar to VACUUM so that the tables are periodically reclustered.
Because the planner records statistics about the ordering of tables, it is advisable to run ANALYZE on the newly clustered table. Otherwise, the planner 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
CREATE TABLE newtable AS SELECT columnlist 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.
Cluster the table employees on the basis of its index emp_ind:
CLUSTER emp_ind ON emp;
Cluster the employees relation using the same index that was used before:
Cluster all the tables on the database that have previously been clustered: