Implement IS NOT DISTINCT FROM, update the regression tests and docs.

Patch from Pavel Stehule, minor fixups by myself.

doc/src/sgml/func.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.297 2005/12/03 16:45:05 momjian Exp $
+$PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.298 2005/12/11 10:54:27 neilc Exp $
 PostgreSQL documentation
 -->
 
@@ -345,16 +345,24 @@ PostgreSQL documentation
     <indexterm>
      <primary>IS DISTINCT FROM</primary>
     </indexterm>
+    <indexterm>
+     <primary>IS NOT DISTINCT FROM</primary>
+    </indexterm>
     The ordinary comparison operators yield null (signifying <quote>unknown</>)
     when either input is null.  Another way to do comparisons is with the
     <literal>IS DISTINCT FROM</literal> construct:
 <synopsis>
 <replaceable>expression</replaceable> IS DISTINCT FROM <replaceable>expression</replaceable>
+<replaceable>expression</replaceable> IS NOT DISTINCT FROM <replaceable>expression</replaceable>
 </synopsis>
-    For non-null inputs this is the same as the <literal>&lt;&gt;</> operator.
-    However, when both inputs are null it will return false, and when just
-    one input is null it will return true.  Thus it effectively acts as though
-    null were a normal data value, rather than <quote>unknown</>.
+    For non-null inputs, <literal>IS DISTINCT FROM</literal> this is
+    the same as the <literal>&lt;&gt;</> operator.  However, when both
+    inputs are null it will return false, and when just one input is
+    null it will return true.  Similarly, <literal>IS NOT DISTINCT
+    FROM</literal> is identical to <literal>=</literal> for non-null
+    inputs, returns true when both inputs are null, and false
+    otherwise. Thus, these constructs effectively act as though null
+    were a normal data value, rather than <quote>unknown</>.
    </para>
 
    <para>

src/backend/parser/gram.y

@@ -11,7 +11,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/parser/gram.y,v 2.516 2005/11/28 04:35:31 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/parser/gram.y,v 2.517 2005/12/11 10:54:27 neilc Exp $
  *
  * HISTORY
  *	  AUTHOR			DATE			MAJOR EVENT
@@ -6715,6 +6715,13 @@ a_expr:		c_expr									{ $$ = $1; }
 				{
 					$$ = (Node *) makeSimpleA_Expr(AEXPR_DISTINCT, "=", $1, $5);
 				}
+			| a_expr IS NOT DISTINCT FROM a_expr		%prec IS
+				{
+					$$ = (Node *) makeA_Expr(AEXPR_NOT, NIL, NULL,
+									(Node *) makeSimpleA_Expr(AEXPR_DISTINCT,
+															  "=", $1, $6));
+
+				}
 			| a_expr IS OF '(' type_list ')'			%prec IS
 				{
 					$$ = (Node *) makeSimpleA_Expr(AEXPR_OF, "=", $1, (Node *) $5);
@@ -6880,6 +6887,11 @@ b_expr:		c_expr
 				{
 					$$ = (Node *) makeSimpleA_Expr(AEXPR_DISTINCT, "=", $1, $5);
 				}
+			| b_expr IS NOT DISTINCT FROM b_expr	%prec IS
+				{
+					$$ = (Node *) makeA_Expr(AEXPR_NOT, NIL,
+						NULL, (Node *) makeSimpleA_Expr(AEXPR_DISTINCT, "=", $1, $6));
+				}
 			| b_expr IS OF '(' type_list ')'	%prec IS
 				{
 					$$ = (Node *) makeSimpleA_Expr(AEXPR_OF, "=", $1, (Node *) $5);

src/test/regress/expected/select_distinct.out

@@ -195,3 +195,28 @@ SELECT null IS DISTINCT FROM null as "no";
  f
 (1 row)
 
+-- ANSI SQL 2003 form
+SELECT 1 IS NOT DISTINCT FROM 2 as "no";
+ no 
+----
+ f
+(1 row)
+
+SELECT 2 IS NOT DISTINCT FROM 2 as "yes";
+ yes 
+-----
+ t
+(1 row)
+
+SELECT 2 IS NOT DISTINCT FROM null as "no";
+ no 
+----
+ f
+(1 row)
+
+SELECT null IS NOT DISTINCT FROM null as "yes";
+ yes 
+-----
+ t
+(1 row)
+

src/test/regress/sql/select_distinct.sql

@@ -56,3 +56,9 @@ SELECT 1 IS DISTINCT FROM 2 as "yes";
 SELECT 2 IS DISTINCT FROM 2 as "no";
 SELECT 2 IS DISTINCT FROM null as "yes";
 SELECT null IS DISTINCT FROM null as "no";
+
+-- ANSI SQL 2003 form
+SELECT 1 IS NOT DISTINCT FROM 2 as "no";
+SELECT 2 IS NOT DISTINCT FROM 2 as "yes";
+SELECT 2 IS NOT DISTINCT FROM null as "no";
+SELECT null IS NOT DISTINCT FROM null as "yes";