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Teach CLUSTER to use seqscan-and-sort when it's faster than indexscan.

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... or at least, when the planner's cost estimates say it will be faster.

Leonardo Francalanci, reviewed by Itagaki Takahiro and Tom Lane
This commit is contained in:
Tom Lane
2010-10-07 20:00:28 -04:00
parent 694c56af2b
commit 3ba11d3df2
14 changed files with 716 additions and 141 deletions
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67
doc/src/sgml/ref/cluster.sgml
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@ -128,18 +128,33 @@ CLUSTER [VERBOSE]
</para>
<para>
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.
<command>CLUSTER</> can re-sort the table using either an indexscan
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.
</para>
<para>
Because <command>CLUSTER</command> remembers the clustering information,
one can cluster the tables one wants clustered manually the first time, and
setup a timed event similar to <command>VACUUM</command> so that the tables
are periodically reclustered.
When an indexscan 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.
</para>
<para>
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 indexscan method, but if the disk space requirement is
intolerable, you can disable this choice by temporarily setting <xref
linkend="guc-enable-sort"> to <literal>off</>.
</para>
<para>
It is advisable to set <xref linkend="guc-maintenance-work-mem"> to
a reasonably large value (but not more than the amount of RAM you can
dedicate to the <command>CLUSTER</> operation) before clustering.
</para>
<para>
@ -150,35 +165,13 @@ CLUSTER [VERBOSE]
</para>
<para>
There is another way to cluster data. 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.)
The other way to cluster a table is to use:
<programlisting>
CREATE TABLE <replaceable class="parameter">newtable</replaceable> AS
SELECT * FROM <replaceable class="parameter">table</replaceable> ORDER BY <replaceable class="parameter">columnlist</replaceable>;
</programlisting>
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.
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 &mdash; 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.
Because <command>CLUSTER</command> 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
<command>CLUSTER</> without any parameters, so that the desired tables
are periodically reclustered.
</para>
</refsect1>
<refsect1>