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Add aggsortop column to pg_aggregate, so that MIN/MAX optimization can

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be supported for all datatypes.  Add CREATE AGGREGATE and pg_dump support
too.  Add specialized min/max aggregates for bpchar, instead of depending
on text's min/max, because otherwise the possible use of bpchar indexes
cannot be recognized.
initdb forced because of catalog changes.
This commit is contained in:
Tom Lane
2005-04-12 04:26:34 +00:00
parent 3803f24379
commit 2e7a68896b
15 changed files with 356 additions and 123 deletions
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39
doc/src/sgml/ref/create_aggregate.sgml
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@@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/ref/create_aggregate.sgml,v 1.31 2005/01/04 00:39:53 tgl Exp $
$PostgreSQL: pgsql/doc/src/sgml/ref/create_aggregate.sgml,v 1.32 2005/04/12 04:26:15 tgl Exp $
PostgreSQL documentation
-->
@@ -26,6 +26,7 @@ CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> (
STYPE = <replaceable class="PARAMETER">state_data_type</replaceable>
[ , FINALFUNC = <replaceable class="PARAMETER">ffunc</replaceable> ]
[ , INITCOND = <replaceable class="PARAMETER">initial_condition</replaceable> ]
[ , SORTOP = <replaceable class="PARAMETER">sort_operator</replaceable> ]
)
</synopsis>
</refsynopsisdiv>
@@ -125,6 +126,29 @@ CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> (
<function>avg</function> returns null when it sees there were zero
input rows.
</para>
<para>
Aggregates that behave like <function>MIN</> or <function>MAX</> can
sometimes be optimized by looking into an index instead of scanning every
input row. If this aggregate can be so optimized, indicate it by
specifying a <firstterm>sort operator</>. The basic requirement is that
the aggregate must yield the first element in the sort ordering induced by
the operator; in other words
<programlisting>
SELECT agg(col) FROM tab;
</programlisting>
must be equivalent to
<programlisting>
SELECT col FROM tab ORDER BY col USING sortop LIMIT 1;
</programlisting>
Further assumptions are that the aggregate ignores null inputs, and that
it delivers a null result if and only if there were no non-null inputs.
Ordinarily, a datatype's <literal>&lt;</> operator is the proper sort
operator for <function>MIN</>, and <literal>&gt;</> is the proper sort
operator for <function>MAX</>. Note that the optimization will never
actually take effect unless the specified operator is the LessThan or
GreaterThan strategy member of a btree index opclass.
</para>
</refsect1>
<refsect1>
@@ -211,6 +235,19 @@ CREATE AGGREGATE <replaceable class="PARAMETER">name</replaceable> (
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">sort_operator</replaceable></term>
<listitem>
<para>
The associated sort operator for a <function>MIN</>- or
<function>MAX</>-like aggregate.
This is just an operator name (possibly schema-qualified).
The operator is assumed to have the same input datatypes as
the aggregate.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>