From e649796f128bd8702ba5744d36f4e8cb81f0b754 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 6 Jun 2003 15:04:03 +0000
Subject: Implement outer-level aggregates to conform to the SQL spec, with
 extensions to support our historical behavior.  An aggregate belongs to the
 closest query level of any of the variables in its argument, or the current
 query level if there are no variables (e.g., COUNT(*)). The implementation
 involves adding an agglevelsup field to Aggref, and treating outer aggregates
 like outer variables at planning time.

---
 doc/src/sgml/syntax.sgml | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

(limited to 'doc/src')

diff --git a/doc/src/sgml/syntax.sgml b/doc/src/sgml/syntax.sgml
index b6ebb1cc434..a1c0767e4cc 100644
--- a/doc/src/sgml/syntax.sgml
+++ b/doc/src/sgml/syntax.sgml
@@ -1,5 +1,5 @@
 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.77 2003/03/25 16:15:38 petere Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/syntax.sgml,v 1.78 2003/06/06 15:04:01 tgl Exp $
 -->
 
 <chapter id="sql-syntax">
@@ -1094,6 +1094,29 @@ sqrt(2)
     linkend="functions-aggregate">.  Other aggregate functions may be added
     by the user. 
    </para>
+
+   <para>
+    An aggregate expression may only appear in the result list or
+    <literal>HAVING</> clause of a <command>SELECT</> command.
+    It is forbidden in other clauses, such as <literal>WHERE</>,
+    because those clauses are logically evaluated before the results
+    of aggregates are formed.
+   </para>
+
+   <para>
+    When an aggregate expression appears in a subquery (see
+    <xref linkend="sql-syntax-scalar-subqueries"> and
+    <xref linkend="functions-subquery">), the aggregate is normally
+    evaluated over the rows of the subquery.  But an exception occurs
+    if the aggregate's argument contains only outer-level variables:
+    the aggregate then belongs to the nearest such outer level, and is
+    evaluated over the rows of that query.  The aggregate expression
+    as a whole is then an outer reference for the subquery it appears in,
+    and acts as a constant over any one evaluation of that subquery.
+    The restriction about
+    appearing only in the result list or <literal>HAVING</> clause
+    applies with respect to the query level that the aggregate belongs to.
+   </para>
   </sect2>
 
   <sect2 id="sql-syntax-type-casts">
@@ -1164,7 +1187,7 @@ CAST ( <replaceable>expression</replaceable> AS <replaceable>type</replaceable>
    </para>
   </sect2>
 
-  <sect2>
+  <sect2 id="sql-syntax-scalar-subqueries">
    <title>Scalar Subqueries</title>
 
    <para>
-- 
cgit v1.2.3