Recognize functional dependency on primary keys. This allows a table's

other columns to be referenced without listing them in GROUP BY, so long as
the primary key column(s) are listed in GROUP BY.

Eventually we should also allow functional dependency on a UNIQUE constraint
when the columns are marked NOT NULL, but that has to wait until NOT NULL
constraints are represented in pg_constraint, because we need to have
pg_constraint OIDs for all the conditions needed to ensure functional
dependency.

Peter Eisentraut, reviewed by Alex Hunsaker and Tom Lane

doc/src/sgml/queries.sgml

@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/queries.sgml,v 1.58 2010/07/03 02:57:46 rhaas Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/queries.sgml,v 1.59 2010/08/07 02:44:05 tgl Exp $ -->
 
 <chapter id="queries">
  <title>Queries</title>
@@ -886,10 +886,7 @@ SELECT product_id, p.name, (sum(s.units) * p.price) AS sales
     In this example, the columns <literal>product_id</literal>,
     <literal>p.name</literal>, and <literal>p.price</literal> must be
     in the <literal>GROUP BY</> clause since they are referenced in
-    the query select list.  (Depending on how the products
-    table is set up, name and price might be fully dependent on the
-    product ID, so the additional groupings could theoretically be
-    unnecessary, though this is not implemented.)  The column
+    the query select list (but see below).  The column
     <literal>s.units</> does not have to be in the <literal>GROUP
     BY</> list since it is only used in an aggregate expression
     (<literal>sum(...)</literal>), which represents the sales
@@ -897,6 +894,18 @@ SELECT product_id, p.name, (sum(s.units) * p.price) AS sales
     all sales of the product.
    </para>
 
+   <para>
+    If the products table is set up so that,
+    say, <literal>product_id</literal> is the primary key, then it
+    would be enough to group by <literal>product_id</literal> in the
+    above example, since name and price would
+    be <firstterm>functionally
+    dependent</firstterm><indexterm><primary>functional
+    dependency</primary></indexterm> on the product ID, and so there
+    would be no ambiguity about which name and price value to return
+    for each product ID group.
+   </para>
+
    <para>
     In strict SQL, <literal>GROUP BY</> can only group by columns of
     the source table but <productname>PostgreSQL</productname> extends

doc/src/sgml/ref/select.sgml

@@ -1,5 +1,5 @@
 <!--
-$PostgreSQL: pgsql/doc/src/sgml/ref/select.sgml,v 1.131 2010/06/15 20:04:53 momjian Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/select.sgml,v 1.132 2010/08/07 02:44:05 tgl Exp $
 PostgreSQL documentation
 -->
 
@@ -520,9 +520,12 @@ GROUP BY <replaceable class="parameter">expression</replaceable> [, ...]
     produces a single value computed across all the selected rows).
     When <literal>GROUP BY</literal> is present, it is not valid for
     the <command>SELECT</command> list expressions to refer to
-    ungrouped columns except within aggregate functions, since there
-    would be more than one possible value to return for an ungrouped
-    column.
+    ungrouped columns except within aggregate functions or if the
+    ungrouped column is functionally dependent on the grouped columns,
+    since there would otherwise be more than one possible value to
+    return for an ungrouped column.  A functional dependency exists if
+    the grouped columns (or a subset thereof) are the primary key of
+    the table containing the ungrouped column.
    </para>
   </refsect2>
 
@@ -1590,6 +1593,18 @@ SELECT distributors.* WHERE distributors.name = 'Westward';
    </para>
   </refsect2>
 
+  <refsect2>
+   <title>Functional Dependencies</title>
+
+   <para>
+    <productname>PostgreSQL</productname> recognizes functional dependency
+    (allowing columns to be omitted from <literal>GROUP BY</>) only when
+    a table's primary key is included in the <literal>GROUP BY</> list.
+    The SQL standard specifies additional conditions that should be
+    recognized.
+   </para>
+  </refsect2>
+
   <refsect2>
    <title><literal>WINDOW</literal> Clause Restrictions</title>