Put back planner's ability to cache the results of mergejoinscansel(),

which I had removed in the first cut of the EquivalenceClass rewrite to
simplify that patch a little.  But it's still important --- in a four-way
join problem mergejoinscansel() was eating about 40% of the planning time
according to gprof.  Also, improve the EquivalenceClass code to re-use
join RestrictInfos rather than generating fresh ones for each join
considered.  This saves some memory space but more importantly improves
the effectiveness of caching planning info in RestrictInfos.

src/include/utils/selfuncs.h

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/selfuncs.h,v 1.38 2007/01/05 22:19:59 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/utils/selfuncs.h,v 1.39 2007/01/22 20:00:40 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -160,7 +160,7 @@ extern Selectivity rowcomparesel(PlannerInfo *root,
 			  int varRelid, JoinType jointype);
 
 extern void mergejoinscansel(PlannerInfo *root, Node *clause,
-				 Oid opfamily, int strategy,
+				 Oid opfamily, int strategy, bool nulls_first,
 				 Selectivity *leftscan,
 				 Selectivity *rightscan);