CLUSTER(7) PostgreSQL 9.5.10 Documentation CLUSTER(7)
NAME
CLUSTER - cluster a table according to an index
SYNOPSIS
CLUSTER [VERBOSE] table_name [ USING index_name ]
CLUSTER [VERBOSE]
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 (ALTER_TABLE(7)) 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.
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
enable_sort to off.
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(7) 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.
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
clusterdb(1)
PostgreSQL 9.5.10 2017 CLUSTER(7)
PostgreSQL 9.5.10 - Generated Thu Nov 30 07:35:06 CST 2017