CLUSTER

CLUSTER

7

SQL - Language Statements

CLUSTER

cluster a table according to an index

CLUSTER \[VERBOSE\]

table\_name

\[ USING

index\_name

\] CLUSTER (

option

\[, \...\] )

table\_name

\[ USING

index\_name

\] CLUSTER \[VERBOSE\]

where

option

can be one of:

VERBOSE \[

boolean

\]

Description
===========

`CLUSTER` instructs PostgreSQL to cluster the table specified by
table\_name based on the index specified by index\_name. The index must
already have been defined on table\_name.

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 the table\'s `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 `CLUSTER table_name` reclusters the table using
the same index as before. You can also use the `CLUSTER` or
`SET WITHOUT CLUSTER` forms of [`ALTER TABLE`](#sql-altertable) 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 form of `CLUSTER` cannot be
executed inside a transaction block.

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.

Parameters
==========

table\_name

:   The name (possibly schema-qualified) of a table.

index\_name

:   The name of an index.

`VERBOSE`

:   Prints a progress report as each table is clustered.

boolean

:   Specifies whether the selected option should be turned on or off.
    You can write `TRUE`, `ON`, or `1` to enable the option, and
    `FALSE`, `OFF`, or `0` to disable it. The boolean value can also be
    omitted, in which case `TRUE` is assumed.

Notes
=====

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 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
information.

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
[???](#guc-enable-sort) to `off`.

It is advisable to set [???](#guc-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`](#sql-analyze) on the newly clustered
table. Otherwise, the planner might make poor choices of query plans.

Because `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.

Each backend running `CLUSTER` will report its progress in the
pg\_stat\_progress\_cluster view. See [???](#cluster-progress-reporting)
for details.

Clustering a partitioned table clusters each of its partitions using the
partition of the specified partitioned index. When clustering a
partitioned table, the index may not be omitted.

Examples
========

Cluster the table `employees` on the basis of its index `employees_ind`:

    CLUSTER employees USING employees_ind;

Cluster the `employees` table using the same index that was used before:

    CLUSTER employees;

Cluster all tables in the database that have previously been clustered:

    CLUSTER;

Compatibility
=============

There is no `CLUSTER` statement in the SQL standard.

The syntax CLUSTER index\_name ON table\_name is also supported for
compatibility with pre-8.3 PostgreSQL versions.

See Also
========
