CLUSTER
SQL - Language Statements
CLUSTER
Gives storage clustering advice to the backend
1998-04-15
CLUSTER indexname ON table
1998-04-15
Inputs
indexname
The name of an index.
table
The name of a table.
1998-04-15
Outputs
CLUSTER
The clustering was done successfully.
ERROR: relation <tablerelation_number> inherits "invoice"
???
This is not documented anywhere. It seems not to be possible to
cluster a table that is inherited.
ERROR: Relation x does not exist!
???
The relation complained of was not shown in the error message,
which contained a random string instead of the relation name.
1998-04-15
Description
This command instructs PostgreSQL to cluster the class specified
by classname approximately
based on the index specified by
indexname. The index must
already have been defined on classname.
When a class is clustered, it is physically reordered
based on the index information. The clustering is static.
In other words, as the class is updated, the changes are
not clustered. No attempt is made to keep new instances or
updated tuples clustered. If he wishes, the user can
recluster manually by issuing the command again.
The table is actually copied to a temporary table in index
order, then renamed back to the original name. For this
reason, all grant permissions and other indexes are lost
when clustering is performed.
In cases where you are accessing single rows randomly
within a table, the actual order of the data in the heap
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 the CLUSTER
command.
Another place CLUSTER is good is in cases where you use an
index to pull out several rows from a table. 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
heap page for the first row that matches, all other rows
that match are probably already on the same heap page,
saving disk accesses and speeding up the query.
There are two ways to cluster data. The first is with the
CLUSTER command, which reorders the original table with
the ordering of the index you specify. This can be slow
on large tables because the rows are fetched from the heap
in index order, and if the heap table is unordered, 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.
Another way is to use
SELECT ... INTO TABLE temp FROM ... ORDER BY ...
This uses the PostgreSQL sorting code in
ORDER BY to match the index, and is much faster for
unordered data. You then drop the old table, use
ALTER TABLE RENAME
to rename 'temp' to the old name, and
recreate the b bindexes. The only problem is that oids
will not be preserved. From then on, CLUSTER should be
fast because most of the heap data has already been
ordered, and the existing index is used.
Usage
Cluster the employees relation on the basis of its salary attribute
CLUSTER emp_ind ON emp
Compatibility
1998-04-15
SQL92
There is no CLUSTER statement in SQL92.