Refactor planner's pathkeys data structure to create a separate, explicit

representation of equivalence classes of variables.  This is an extensive
rewrite, but it brings a number of benefits:
* planner no longer fails in the presence of "incomplete" operator families
that don't offer operators for every possible combination of datatypes.
* avoid generating and then discarding redundant equality clauses.
* remove bogus assumption that derived equalities always use operators
named "=".
* mergejoins can work with a variety of sort orders (e.g., descending) now,
instead of tying each mergejoinable operator to exactly one sort order.
* better recognition of redundant sort columns.
* can make use of equalities appearing underneath an outer join.

src/include/optimizer/pathnode.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/optimizer/pathnode.h,v 1.75 2007/01/10 18:06:04 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/optimizer/pathnode.h,v 1.76 2007/01/20 20:45:40 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -71,9 +71,6 @@ extern MergePath *create_mergejoin_path(PlannerInfo *root,
 					  List *restrict_clauses,
 					  List *pathkeys,
 					  List *mergeclauses,
-					  Oid *mergefamilies,
-					  int *mergestrategies,
-					  bool *mergenullsfirst,
 					  List *outersortkeys,
 					  List *innersortkeys);