|PostgreSQL Reference Manual - Volume 1 - SQL Language Reference|
by The PostgreSQL Global Development Group
Paperback (6"x9"), 716 pages
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CLUSTER -- cluster a table according to an index
CLUSTER indexname ON tablename CLUSTER tablename CLUSTER
CLUSTER instructs PostgreSQL
to cluster the table specified
based on the index specified by
indexname. 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.
When a table is clustered, PostgreSQL
remembers on which index it was clustered. The form
reclusters the table on the same index that it was clustered before.
CLUSTER without any parameter reclusters all the tables
current database that the calling user owns, or all tables if called
by a superuser. (Never-clustered tables are not included.) 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 of an index.
- The name (possibly schema-qualified) of a table.
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.
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.
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 by
scanning it using the index you specify. This can be slow
on large tables because the rows are fetched from the table
in index order, and if the table is disordered, 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 * FROM table ORDER BY columnlist;
which uses the PostgreSQL sorting code
to produce the desired order;
this is usually much faster than an index scan for disordered data.
Then you drop the old table, use
ALTER TABLE ... RENAME
to rename newtable to the
old name, and recreate the table's indexes.
The big disadvantage of this approach is that it does not preserve
OIDs, constraints, foreign key relationships, granted privileges, and
other ancillary properties of the table--all such items must be
manually recreated. Another disadvantage is that this way requires a sort
temporary file about the same size as the table itself, so peak disk usage
is about three times the table size instead of twice the table size.
Cluster the table
employees on the basis of
CLUSTER emp_ind ON emp;
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.
|ISBN 0954612027||PostgreSQL Reference Manual - Volume 1 - SQL Language Reference||See the print edition|