index fd247be28c2a905e6526e742b78f943dd2f4d10d..b81dc923871973eb78ffede94d73974aa396fa82 100644 (file)
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-$Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.140 2003年02月16日 02:30:36 tgl Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.141 2003年02月19日 03:12:22 momjian Exp $
PostgreSQL documentation
-->
@@ -7074,6 +7074,57 @@ SELECT pg_type_is_visible('myschema.widget'::regtype);
used to substitute zero for null when necessary.
</para>
+ <note>
+ <para>
+ Users accustomed to working with other RDBMS products 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
+<programlisting>
+SELECT min(col) 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 <quote>special-casing</quote> 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 query that can be
+ used to improve the performance of <function>count()</function>
+ when applied to the entire table.
+ </para>
+ </note>
+
</sect1>