Index: doc/src/sgml/func.sgml
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RCS file: /var/lib/cvs/pgsql-server/doc/src/sgml/func.sgml,v
retrieving revision 1.136
diff -c -r1.136 func.sgml
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--- doc/src/sgml/func.sgml	2 Feb 2003 03:25:47 -0000
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     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>