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/optimizer/path/costsize.c

@@ -42,7 +42,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.88 2002/06/26 21:58:56 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.89 2002/07/04 15:23:56 thomas Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1092,6 +1092,7 @@ cost_qual_eval_walker(Node *node, Cost *total)
 		switch (expr->opType)
 		{
 			case OP_EXPR:
+			case DISTINCT_EXPR:
 			case FUNC_EXPR:
 				*total += cpu_operator_cost;
 				break;