CLUSTER — cluster a table according to an index
index_name] CLUSTER [VERBOSE]
CLUSTER instructs PostgreSQL to cluster the table specified
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 the table's
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
the table using the same index as before. You can also use the
SET WITHOUT CLUSTER forms of ALTER TABLE to set the index to be
used for future cluster operations, or to clear any previous
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
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 (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
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
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
the basis of its index
CLUSTER employees USING employees_ind;
using the same index that was used before:
Cluster all tables in the database that have previously been clustered:
There is no
in the SQL standard.
is also supported for compatibility with pre-8.3 PostgreSQL versions.
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