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| -rw-r--r-- | doc/src/sgml/func.sgml | 53 |
1 files changed, 52 insertions, 1 deletions
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml index fd247be28c2..b81dc923871 100644 --- a/doc/src/sgml/func.sgml +++ b/doc/src/sgml/func.sgml @@ -1,5 +1,5 @@ <!-- -$Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.140 2003/02/16 02:30:36 tgl Exp $ +$Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.141 2003/02/19 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> |