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Remove some verbiage describing how min() and max() are slow when applied

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to the entire table: as of current sources, they are no longer slow
provided there is an index on the column.
This commit is contained in:
Neil Conway 2005-05-23 01:50:01 +00:00
parent dd2894dfd6
commit 1b41965d5d
1 changed files with 6 additions and 45 deletions
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51
doc/src/sgml/func.sgml
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@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.249 2005/05/23 01:29:54 neilc Exp $
$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.250 2005/05/23 01:50:01 neilc Exp $
PostgreSQL documentation
-->
@ -7317,55 +7317,16 @@ SELECT b1 = ANY((SELECT b2 FROM t2 ...)) FROM t1 ...;
<note>
<para>
Users accustomed to working with other SQL database management
systems may be surprised by the performance characteristics of
certain aggregate functions in
<productname>PostgreSQL</productname> when the aggregate is
applied to the entire table (in other words, no
<literal>WHERE</literal> clause is specified). In particular, a
query like
systems may be surprised by the performance of the
<function>count</function> aggregate when it is applied to the
entire table. A query like:
<programlisting>
SELECT min(col) FROM sometable;
SELECT count(*) FROM sometable;
</programlisting>
will be executed by <productname>PostgreSQL</productname> using a
sequential scan of the entire table. Other database systems may
optimize queries of this form to use an index on the column, if
one is available. Similarly, the aggregate functions
<function>max()</function> and <function>count()</function> always
require a sequential scan if applied to the entire table in
<productname>PostgreSQL</productname>.
</para>
<para>
<productname>PostgreSQL</productname> cannot easily implement this
optimization because it also allows for user-defined aggregate
queries. Since <function>min()</function>,
<function>max()</function>, and <function>count()</function> are
defined using a generic API for aggregate functions, there is no
provision for special-casing the execution of these functions
under certain circumstances.
</para>
<para>
Fortunately, there is a simple workaround for
<function>min()</function> and <function>max()</function>. The
query shown below is equivalent to the query above, except that it
can take advantage of a B-tree index if there is one present on
the column in question.
<programlisting>
SELECT col FROM sometable ORDER BY col ASC LIMIT 1;
</programlisting>
A similar query (obtained by substituting <literal>DESC</literal>
for <literal>ASC</literal> in the query above) can be used in the
place of <function>max()</function>.
</para>
<para>
Unfortunately, there is no similarly trivial workaround that can
be used to improve the performance of <function>count()</function>
when applied to the entire table.
sequential scan of the entire table.
</para>
</note>
</sect1>