If you are requesting a range of indexed values from a table, or a
single indexed value that has multiple rows that match,
<command>CLUSTER</command> 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,
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.
</para>
...
...
@@ -137,30 +137,33 @@ CLUSTER
<para>
There is another way to cluster data. The
<command>CLUSTER</command> command reorders the original table using
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
<command>CLUSTER</command> 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. (<productname>PostgreSQL</productname> has a cache,
but the majority of a big table will not fit in the cache.)
retrieved for every row moved. (<productname>PostgreSQL</productname> 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
<programlisting>
CREATE TABLE <replaceable class="parameter">newtable</replaceable> AS
SELECT <replaceable class="parameter">columnlist</replaceable> FROM <replaceable class="parameter">table</replaceable> ORDER BY <replaceable class="parameter">columnlist</replaceable>;
SELECT * FROM <replaceable class="parameter">table</replaceable> ORDER BY <replaceable class="parameter">columnlist</replaceable>;
</programlisting>
which uses the <productname>PostgreSQL</productname> sorting code in
the <literal>ORDER BY</literal> clause to create the desired order; this is usually much
faster than an index scan for
unordered data. You then drop the old table, use
which uses the <productname>PostgreSQL</productname> 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
<command>ALTER TABLE ... RENAME</command>
to rename <replaceable class="parameter">newtable</replaceable> to the old name, and
recreate the table's indexes. However, this approach does not preserve
to rename <replaceable class="parameter">newtable</replaceable> 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.
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.
