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+$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.250 2005年05月23日 01:50:01 neilc Exp $
PostgreSQL documentation
-->
<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>