Teach planagg.c that partial indexes specifying WHERE foo IS NOT NULL can be

used to perform MIN(foo) or MAX(foo), since we want to discard null rows in
the indexscan anyway.  (This would probably fall out for free if we were
injecting the IS NOT NULL clause somewhere earlier, but given the current
anatomy of the MIN/MAX optimization code we have to do it explicitly.
Fortunately, very little added code is needed.)  Per a discussion with
Henk de Wit.

src/backend/optimizer/plan/planagg.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planagg.c,v 1.32 2007/04/27 22:05:47 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planagg.c,v 1.33 2007/10/13 00:58:03 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -22,6 +22,7 @@
 #include "optimizer/pathnode.h"
 #include "optimizer/paths.h"
 #include "optimizer/planmain.h"
+#include "optimizer/predtest.h"
 #include "optimizer/subselect.h"
 #include "parser/parse_clause.h"
 #include "parser/parse_expr.h"
@@ -35,6 +36,7 @@ typedef struct
 	Oid			aggfnoid;		/* pg_proc Oid of the aggregate */
 	Oid			aggsortop;		/* Oid of its sort operator */
 	Expr	   *target;			/* expression we are aggregating on */
+	Expr	   *notnulltest;	/* expression for "target IS NOT NULL" */
 	IndexPath  *path;			/* access path for index scan */
 	Cost		pathcost;		/* estimated cost to fetch first row */
 	bool		nulls_first;	/* null ordering direction matching index */
@@ -285,8 +287,23 @@ build_minmax_path(PlannerInfo *root, RelOptInfo *rel, MinMaxAggInfo *info)
 	IndexPath  *best_path = NULL;
 	Cost		best_cost = 0;
 	bool		best_nulls_first = false;
+	NullTest   *ntest;
+	List	   *allquals;
 	ListCell   *l;
 
+	/* Build "target IS NOT NULL" expression for use below */
+	ntest = makeNode(NullTest);
+	ntest->nulltesttype = IS_NOT_NULL;
+	ntest->arg = copyObject(info->target);
+	info->notnulltest = (Expr *) ntest;
+
+	/*
+	 * Build list of existing restriction clauses plus the notnull test.
+	 * We cheat a bit by not bothering with a RestrictInfo node for the
+	 * notnull test --- predicate_implied_by() won't care.
+	 */
+	allquals = list_concat(list_make1(ntest), rel->baserestrictinfo);
+
 	foreach(l, rel->indexlist)
 	{
 		IndexOptInfo *index = (IndexOptInfo *) lfirst(l);
@@ -302,8 +319,13 @@ build_minmax_path(PlannerInfo *root, RelOptInfo *rel, MinMaxAggInfo *info)
 		if (index->relam != BTREE_AM_OID)
 			continue;
 
-		/* Ignore partial indexes that do not match the query */
-		if (index->indpred != NIL && !index->predOK)
+		/*
+		 * Ignore partial indexes that do not match the query --- unless
+		 * their predicates can be proven from the baserestrict list plus
+		 * the IS NOT NULL test.  In that case we can use them.
+		 */
+		if (index->indpred != NIL && !index->predOK &&
+			!predicate_implied_by(index->indpred, allquals))
 			continue;
 
 		/*
@@ -441,7 +463,6 @@ make_agg_subplan(PlannerInfo *root, MinMaxAggInfo *info)
 	Plan	   *iplan;
 	TargetEntry *tle;
 	SortClause *sortcl;
-	NullTest   *ntest;
 
 	/*
 	 * Generate a suitably modified query.	Much of the work here is probably
@@ -487,7 +508,7 @@ make_agg_subplan(PlannerInfo *root, MinMaxAggInfo *info)
 	 * basic indexscan, but we have to convert it to a Plan and attach a LIMIT
 	 * node above it.
 	 *
-	 * Also we must add a "WHERE foo IS NOT NULL" restriction to the
+	 * Also we must add a "WHERE target IS NOT NULL" restriction to the
 	 * indexscan, to be sure we don't return a NULL, which'd be contrary to
 	 * the standard behavior of MIN/MAX.  XXX ideally this should be done
 	 * earlier, so that the selectivity of the restriction could be included
@@ -497,6 +518,9 @@ make_agg_subplan(PlannerInfo *root, MinMaxAggInfo *info)
 	 * The NOT NULL qual has to go on the actual indexscan; create_plan might
 	 * have stuck a gating Result atop that, if there were any pseudoconstant
 	 * quals.
+	 *
+	 * We can skip adding the NOT NULL qual if it's redundant with either
+	 * an already-given WHERE condition, or a clause of the index predicate.
 	 */
 	plan = create_plan(&subroot, (Path *) info->path);
 
@@ -508,11 +532,9 @@ make_agg_subplan(PlannerInfo *root, MinMaxAggInfo *info)
 		iplan = plan;
 	Assert(IsA(iplan, IndexScan));
 
-	ntest = makeNode(NullTest);
-	ntest->nulltesttype = IS_NOT_NULL;
-	ntest->arg = copyObject(info->target);
-
-	iplan->qual = lcons(ntest, iplan->qual);
+	if (!list_member(iplan->qual, info->notnulltest) &&
+		!list_member(info->path->indexinfo->indpred, info->notnulltest))
+		iplan->qual = lcons(info->notnulltest, iplan->qual);
 
 	plan = (Plan *) make_limit(plan,
 							   subparse->limitOffset,