Restructure planning of nestloop inner indexscans so that the set of usable

joinclauses is determined accurately for each join.  Formerly, the code only
considered joinclauses that used all of the rels from the outer side of the
join; thus for example
	FROM (a CROSS JOIN b) JOIN c ON (c.f1 = a.x AND c.f2 = b.y)
could not exploit a two-column index on c(f1,f2), since neither of the
qual clauses would be in the joininfo list it looked in.  The new code does
this correctly, and also is able to eliminate redundant clauses, thus fixing
the problem noted 24-Oct-02 by Hans-Jürgen Schönig.

src/backend/optimizer/util/restrictinfo.c

@@ -8,21 +8,21 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/optimizer/util/restrictinfo.c,v 1.14 2002/06/20 20:29:31 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/optimizer/util/restrictinfo.c,v 1.15 2002/11/24 21:52:14 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
 
-
 #include "optimizer/clauses.h"
+#include "optimizer/paths.h"
 #include "optimizer/restrictinfo.h"
 
+
 /*
  * restriction_is_or_clause
  *
  * Returns t iff the restrictinfo node contains an 'or' clause.
- *
  */
 bool
 restriction_is_or_clause(RestrictInfo *restrictinfo)
@@ -37,8 +37,7 @@ restriction_is_or_clause(RestrictInfo *restrictinfo)
 /*
  * get_actual_clauses
  *
- * Returns a list containing the clauses from 'restrictinfo_list'.
- *
+ * Returns a list containing the bare clauses from 'restrictinfo_list'.
  */
 List *
 get_actual_clauses(List *restrictinfo_list)
@@ -80,3 +79,81 @@ get_actual_join_clauses(List *restrictinfo_list,
 			*joinquals = lappend(*joinquals, clause->clause);
 	}
 }
+
+/*
+ * remove_redundant_join_clauses
+ *
+ * Given a list of RestrictInfo clauses that are to be applied in a join,
+ * remove any duplicate or redundant clauses.
+ *
+ * We must eliminate duplicates when forming the restrictlist for a joinrel,
+ * since we will see many of the same clauses arriving from both input
+ * relations. Also, if a clause is a mergejoinable clause, it's possible that
+ * it is redundant with previous clauses (see optimizer/README for
+ * discussion). We detect that case and omit the redundant clause from the
+ * result list.
+ *
+ * We can detect redundant mergejoinable clauses very cheaply by using their
+ * left and right pathkeys, which uniquely identify the sets of equijoined
+ * variables in question.  All the members of a pathkey set that are in the
+ * left relation have already been forced to be equal; likewise for those in
+ * the right relation.  So, we need to have only one clause that checks
+ * equality between any set member on the left and any member on the right;
+ * by transitivity, all the rest are then equal.
+ *
+ * Weird special case: if we have two clauses that seem redundant
+ * except one is pushed down into an outer join and the other isn't,
+ * then they're not really redundant, because one constrains the
+ * joined rows after addition of null fill rows, and the other doesn't.
+ *
+ * The result is a fresh List, but it points to the same member nodes
+ * as were in the input.
+ */
+List *
+remove_redundant_join_clauses(Query *root, List *restrictinfo_list,
+							  JoinType jointype)
+{
+	List	   *result = NIL;
+	List	   *item;
+
+	foreach(item, restrictinfo_list)
+	{
+		RestrictInfo *rinfo = (RestrictInfo *) lfirst(item);
+
+		/* eliminate duplicates */
+		if (member(rinfo, result))
+			continue;
+
+		/* check for redundant merge clauses */
+		if (rinfo->mergejoinoperator != InvalidOid)
+		{
+			bool		redundant = false;
+			List	   *olditem;
+
+			cache_mergeclause_pathkeys(root, rinfo);
+
+			foreach(olditem, result)
+			{
+				RestrictInfo *oldrinfo = (RestrictInfo *) lfirst(olditem);
+
+				if (oldrinfo->mergejoinoperator != InvalidOid &&
+					rinfo->left_pathkey == oldrinfo->left_pathkey &&
+					rinfo->right_pathkey == oldrinfo->right_pathkey &&
+					(rinfo->ispusheddown == oldrinfo->ispusheddown ||
+					 !IS_OUTER_JOIN(jointype)))
+				{
+					redundant = true;
+					break;
+				}
+			}
+
+			if (redundant)
+				continue;
+		}
+
+		/* otherwise, add it to result list */
+		result = lappend(result, rinfo);
+	}
+
+	return result;
+}