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/nodes/nodes.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/nodes/nodes.h,v 1.191 2007/01/05 22:19:55 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/nodes.h,v 1.192 2007/01/20 20:45:40 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -190,7 +190,9 @@ typedef enum NodeTag
 	T_ResultPath,
 	T_MaterialPath,
 	T_UniquePath,
-	T_PathKeyItem,
+	T_EquivalenceClass,
+	T_EquivalenceMember,
+	T_PathKey,
 	T_RestrictInfo,
 	T_InnerIndexscanInfo,
 	T_OuterJoinInfo,