Implement the IS DISTINCT FROM operator per SQL99.

Reused the Expr node to hold DISTINCT which strongly resembles
 the existing OP info. Define DISTINCT_EXPR which strongly resembles
 the existing OPER_EXPR opType, but with handling for NULLs required
 by SQL99.
We have explicit support for single-element DISTINCT comparisons
 all the way through to the executor. But, multi-element DISTINCTs
 are handled by expanding into a comparison tree in gram.y as is done for
 other row comparisons. Per discussions, it might be desirable to move
 this into one or more purpose-built nodes to be handled in the backend.
Define the optional ROW keyword and token per SQL99.
 This allows single-element row constructs, which were formerly disallowed
 due to shift/reduce conflicts with parenthesized a_expr clauses.
Define the SQL99 TREAT() function. Currently, use as a synonym for CAST().

src/backend/parser/keywords.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/parser/keywords.c,v 1.117 2002/06/22 02:04:45 thomas Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/parser/keywords.c,v 1.118 2002/07/04 15:24:01 thomas Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -283,6 +283,7 @@ static const ScanKeyword ScanKeywords[] = {
 	{"toast", TOAST},
 	{"trailing", TRAILING},
 	{"transaction", TRANSACTION},
+	{"treat", TREAT},
 	{"trigger", TRIGGER},
 	{"trim", TRIM},
 	{"true", TRUE_P},