CLUSTER — cluster a table according to an index
index_name] CLUSTER [VERBOSE]
CLUSTER instructs PostgreSQL
to cluster the table specified
based on the index specified by
index_name. The index must
already have been defined on
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. Also, setting
fillfactor storage parameter to less than
100% can aid in preserving cluster ordering during updates, since updated
rows are kept on the same page if enough space is available there.)
When a table is clustered, PostgreSQL
remembers which index it was clustered by. The form
reclusters the table using the same index as before. You can also
SET WITHOUT CLUSTER
forms of ALTER TABLE to set the index to be used for
future cluster operations, or to clear any previous setting.
CLUSTER without any parameter reclusters all the
previously-clustered tables in the current database that the calling user
owns, or all such tables if called by a superuser. This
CLUSTER cannot be executed inside a transaction
When a table is being clustered, an
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 (possibly schema-qualified) of a table.
The name of an index.
Prints a progress report as each table is clustered.
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
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
table page for the first row that matches, all other rows
that match are probably already on the same table page,
and so you save disk accesses and speed up the query.
CLUSTER can re-sort the table using either an index scan
on the specified index, or (if the index is a b-tree) a sequential
scan followed by sorting. It will attempt to choose the method that
will be faster, based on planner cost parameters and available statistical
When an index scan is used, 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.
When a sequential scan and sort is used, a temporary sort file is
also created, so that the peak temporary space requirement is as much
as double the table size, plus the index sizes. This method is often
faster than the index scan method, but if the disk space requirement is
intolerable, you can disable this choice by temporarily setting enable_sort to
It is advisable to set maintenance_work_mem to
a reasonably large value (but not more than the amount of RAM you can
dedicate to the
CLUSTER operation) before clustering.
Because the planner records statistics about the ordering of tables, it is advisable to run ANALYZE on the newly clustered table. Otherwise, the planner might make poor choices of query plans.
CLUSTER remembers which indexes are clustered,
one can cluster the tables one wants clustered manually the first time,
then set up a periodic maintenance script that executes
CLUSTER without any parameters, so that the desired tables
are periodically reclustered.
Cluster the table
employees on the basis of
CLUSTER employees USING employees_ind;
employees table using the same
index that was used before:
Cluster all tables in the database that have previously been clustered:
There is no
CLUSTER statement in the SQL standard.
is also supported for compatibility with pre-8.3 PostgreSQL versions.