When estimating the selectivity of an inequality "column > constant" or

"column < constant", and the comparison value is in the first or last
histogram bin or outside the histogram entirely, try to fetch the actual
column min or max value using an index scan (if there is an index on the
column).  If successful, replace the lower or upper histogram bound with
that value before carrying on with the estimate.  This limits the
estimation error caused by moving min/max values when the comparison
value is close to the min or max.  Per a complaint from Josh Berkus.

It is tempting to consider using this mechanism for mergejoinscansel as well,
but that would inject index fetches into main-line join estimation not just
endpoint cases.  I'm refraining from that until we can get a better handle
on the costs of doing this type of lookup.

src/backend/executor/nodeHash.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeHash.c,v 1.125 2010/01/02 16:57:41 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeHash.c,v 1.126 2010/01/04 02:44:39 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1006,6 +1006,7 @@ ExecHashBuildSkewHash(HashJoinTable hashtable, Hash *node, int mcvsToUse)
 
 	if (get_attstatsslot(statsTuple, node->skewColType, node->skewColTypmod,
 						 STATISTIC_KIND_MCV, InvalidOid,
+						 NULL,
 						 &values, &nvalues,
 						 &numbers, &nnumbers))
 	{

src/backend/tsearch/ts_selfuncs.c

@@ -7,7 +7,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/tsearch/ts_selfuncs.c,v 1.6 2010/01/02 16:57:53 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/tsearch/ts_selfuncs.c,v 1.7 2010/01/04 02:44:39 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -170,6 +170,7 @@ tsquerysel(VariableStatData *vardata, Datum constval)
 		if (get_attstatsslot(vardata->statsTuple,
 							 TEXTOID, -1,
 							 STATISTIC_KIND_MCELEM, InvalidOid,
+							 NULL,
 							 &values, &nvalues,
 							 &numbers, &nnumbers))
 		{

src/backend/utils/adt/selfuncs.c

@@ -15,7 +15,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.266 2010/01/02 16:57:55 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.267 2010/01/04 02:44:39 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -92,9 +92,11 @@
 #include <math.h>
 
 #include "access/sysattr.h"
+#include "catalog/index.h"
 #include "catalog/pg_opfamily.h"
 #include "catalog/pg_statistic.h"
 #include "catalog/pg_type.h"
+#include "executor/executor.h"
 #include "mb/pg_wchar.h"
 #include "nodes/makefuncs.h"
 #include "nodes/nodeFuncs.h"
@@ -118,6 +120,7 @@
 #include "utils/pg_locale.h"
 #include "utils/selfuncs.h"
 #include "utils/syscache.h"
+#include "utils/tqual.h"
 
 
 /* Hooks for plugins to get control when we ask for stats */
@@ -130,7 +133,8 @@ static double var_eq_const(VariableStatData *vardata, Oid operator,
 static double var_eq_non_const(VariableStatData *vardata, Oid operator,
 				 Node *other,
 				 bool varonleft);
-static double ineq_histogram_selectivity(VariableStatData *vardata,
+static double ineq_histogram_selectivity(PlannerInfo *root,
+						   VariableStatData *vardata,
 						   FmgrInfo *opproc, bool isgt,
 						   Datum constval, Oid consttype);
 static double eqjoinsel_inner(Oid operator,
@@ -161,7 +165,12 @@ static char *convert_string_datum(Datum value, Oid typid);
 static double convert_timevalue_to_scalar(Datum value, Oid typid);
 static bool get_variable_range(PlannerInfo *root, VariableStatData *vardata,
 				   Oid sortop, Datum *min, Datum *max);
-static Selectivity prefix_selectivity(VariableStatData *vardata,
+static bool get_actual_variable_range(PlannerInfo *root,
+									  VariableStatData *vardata,
+									  Oid sortop,
+									  Datum *min, Datum *max);
+static Selectivity prefix_selectivity(PlannerInfo *root,
+				   VariableStatData *vardata,
 				   Oid vartype, Oid opfamily, Const *prefixcon);
 static Selectivity pattern_selectivity(Const *patt, Pattern_Type ptype);
 static Datum string_to_datum(const char *str, Oid datatype);
@@ -266,6 +275,7 @@ var_eq_const(VariableStatData *vardata, Oid operator,
 		if (get_attstatsslot(vardata->statsTuple,
 							 vardata->atttype, vardata->atttypmod,
 							 STATISTIC_KIND_MCV, InvalidOid,
+							 NULL,
 							 &values, &nvalues,
 							 &numbers, &nnumbers))
 		{
@@ -405,6 +415,7 @@ var_eq_non_const(VariableStatData *vardata, Oid operator,
 		if (get_attstatsslot(vardata->statsTuple,
 							 vardata->atttype, vardata->atttypmod,
 							 STATISTIC_KIND_MCV, InvalidOid,
+							 NULL,
 							 NULL, NULL,
 							 &numbers, &nnumbers))
 		{
@@ -514,7 +525,7 @@ scalarineqsel(PlannerInfo *root, Oid operator, bool isgt,
 	 * If there is a histogram, determine which bin the constant falls in, and
 	 * compute the resulting contribution to selectivity.
 	 */
-	hist_selec = ineq_histogram_selectivity(vardata, &opproc, isgt,
+	hist_selec = ineq_histogram_selectivity(root, vardata, &opproc, isgt,
 											constval, consttype);
 
 	/*
@@ -524,7 +535,7 @@ scalarineqsel(PlannerInfo *root, Oid operator, bool isgt,
 	 */
 	selec = 1.0 - stats->stanullfrac - sumcommon;
 
-	if (hist_selec > 0.0)
+	if (hist_selec >= 0.0)
 		selec *= hist_selec;
 	else
 	{
@@ -575,6 +586,7 @@ mcv_selectivity(VariableStatData *vardata, FmgrInfo *opproc,
 		get_attstatsslot(vardata->statsTuple,
 						 vardata->atttype, vardata->atttypmod,
 						 STATISTIC_KIND_MCV, InvalidOid,
+						 NULL,
 						 &values, &nvalues,
 						 &numbers, &nnumbers))
 	{
@@ -648,6 +660,7 @@ histogram_selectivity(VariableStatData *vardata, FmgrInfo *opproc,
 		get_attstatsslot(vardata->statsTuple,
 						 vardata->atttype, vardata->atttypmod,
 						 STATISTIC_KIND_HISTOGRAM, InvalidOid,
+						 NULL,
 						 &values, &nvalues,
 						 NULL, NULL))
 	{
@@ -689,23 +702,24 @@ histogram_selectivity(VariableStatData *vardata, FmgrInfo *opproc,
  * Determine the fraction of the variable's histogram population that
  * satisfies the inequality condition, ie, VAR < CONST or VAR > CONST.
  *
- * Returns zero if there is no histogram (valid results will always be
- * greater than zero).
+ * Returns -1 if there is no histogram (valid results will always be >= 0).
  *
  * Note that the result disregards both the most-common-values (if any) and
  * null entries.  The caller is expected to combine this result with
  * statistics for those portions of the column population.
  */
 static double
-ineq_histogram_selectivity(VariableStatData *vardata,
+ineq_histogram_selectivity(PlannerInfo *root,
+						   VariableStatData *vardata,
 						   FmgrInfo *opproc, bool isgt,
 						   Datum constval, Oid consttype)
 {
 	double		hist_selec;
+	Oid			hist_op;
 	Datum	   *values;
 	int			nvalues;
 
-	hist_selec = 0.0;
+	hist_selec = -1.0;
 
 	/*
 	 * Someday, ANALYZE might store more than one histogram per rel/att,
@@ -721,6 +735,7 @@ ineq_histogram_selectivity(VariableStatData *vardata,
 		get_attstatsslot(vardata->statsTuple,
 						 vardata->atttype, vardata->atttypmod,
 						 STATISTIC_KIND_HISTOGRAM, InvalidOid,
+						 &hist_op,
 						 &values, &nvalues,
 						 NULL, NULL))
 	{
@@ -732,16 +747,56 @@ ineq_histogram_selectivity(VariableStatData *vardata,
 			 * actually sort-compatible with the histogram, you'll get garbage
 			 * results ... but probably not any more garbage-y than you would
 			 * from the old linear search.)
+			 *
+			 * If the binary search accesses the first or last histogram entry,
+			 * we try to replace that endpoint with the true column min or max
+			 * as found by get_actual_variable_range().  This ameliorates
+			 * misestimates when the min or max is moving as a result of
+			 * changes since the last ANALYZE.  Note that this could result
+			 * in effectively including MCVs into the histogram that weren't
+			 * there before, but we don't try to correct for that.
 			 */
 			double		histfrac;
 			int			lobound = 0;	/* first possible slot to search */
 			int			hibound = nvalues;		/* last+1 slot to search */
+			bool		have_end = false;
+
+			/*
+			 * If there are only two histogram entries, we'll want up-to-date
+			 * values for both.  (If there are more than two, we need at most
+			 * one of them to be updated, so we deal with that within the
+			 * loop.)
+			 */
+			if (nvalues == 2)
+				have_end = get_actual_variable_range(root,
+													 vardata,
+													 hist_op,
+													 &values[0],
+													 &values[1]);
 
 			while (lobound < hibound)
 			{
 				int			probe = (lobound + hibound) / 2;
 				bool		ltcmp;
 
+				/*
+				 * If we find ourselves about to compare to the first or last
+				 * histogram entry, first try to replace it with the actual
+				 * current min or max (unless we already did so above).
+				 */
+				if (probe == 0 && nvalues > 2)
+					have_end = get_actual_variable_range(root,
+														 vardata,
+														 hist_op,
+														 &values[0],
+														 NULL);
+				else if (probe == nvalues - 1 && nvalues > 2)
+					have_end = get_actual_variable_range(root,
+														 vardata,
+														 hist_op,
+														 NULL,
+														 &values[probe]);
+
 				ltcmp = DatumGetBool(FunctionCall2(opproc,
 												   values[probe],
 												   constval));
@@ -772,7 +827,7 @@ ineq_histogram_selectivity(VariableStatData *vardata,
 				double		binfrac;
 
 				/*
-				 * We have values[i-1] < constant < values[i].
+				 * We have values[i-1] <= constant <= values[i].
 				 *
 				 * Convert the constant and the two nearest bin boundary
 				 * values to a uniform comparison scale, and do a linear
@@ -840,13 +895,19 @@ ineq_histogram_selectivity(VariableStatData *vardata,
 			/*
 			 * The histogram boundaries are only approximate to begin with,
 			 * and may well be out of date anyway.	Therefore, don't believe
-			 * extremely small or large selectivity estimates.
+			 * extremely small or large selectivity estimates --- unless we
+			 * got actual current endpoint values from the table.
 			 */
+			if (have_end)
+				CLAMP_PROBABILITY(hist_selec);
+			else
+			{
 				if (hist_selec < 0.0001)
 					hist_selec = 0.0001;
 				else if (hist_selec > 0.9999)
 					hist_selec = 0.9999;
 			}
+		}
 
 		free_attstatsslot(vardata->atttype, values, nvalues, NULL, 0);
 	}
@@ -1198,7 +1259,7 @@ patternsel(PG_FUNCTION_ARGS, Pattern_Type ptype, bool negate)
 			Selectivity restsel;
 
 			if (pstatus == Pattern_Prefix_Partial)
-				prefixsel = prefix_selectivity(&vardata, vartype,
+				prefixsel = prefix_selectivity(root, &vardata, vartype,
 											   opfamily, prefix);
 			else
 				prefixsel = 1.0;
@@ -1363,6 +1424,7 @@ booltestsel(PlannerInfo *root, BoolTestType booltesttype, Node *arg,
 		if (get_attstatsslot(vardata.statsTuple,
 							 vardata.atttype, vardata.atttypmod,
 							 STATISTIC_KIND_MCV, InvalidOid,
+							 NULL,
 							 &values, &nvalues,
 							 &numbers, &nnumbers)
 			&& nnumbers > 0)
@@ -1999,6 +2061,7 @@ eqjoinsel_inner(Oid operator,
 									  vardata1->atttypmod,
 									  STATISTIC_KIND_MCV,
 									  InvalidOid,
+									  NULL,
 									  &values1, &nvalues1,
 									  &numbers1, &nnumbers1);
 	}
@@ -2011,6 +2074,7 @@ eqjoinsel_inner(Oid operator,
 									  vardata2->atttypmod,
 									  STATISTIC_KIND_MCV,
 									  InvalidOid,
+									  NULL,
 									  &values2, &nvalues2,
 									  &numbers2, &nnumbers2);
 	}
@@ -2232,6 +2296,7 @@ eqjoinsel_semi(Oid operator,
 									  vardata1->atttypmod,
 									  STATISTIC_KIND_MCV,
 									  InvalidOid,
+									  NULL,
 									  &values1, &nvalues1,
 									  &numbers1, &nnumbers1);
 	}
@@ -2244,6 +2309,7 @@ eqjoinsel_semi(Oid operator,
 									  vardata2->atttypmod,
 									  STATISTIC_KIND_MCV,
 									  InvalidOid,
+									  NULL,
 									  &values2, &nvalues2,
 									  &numbers2, &nnumbers2);
 	}
@@ -3226,7 +3292,9 @@ estimate_hash_bucketsize(PlannerInfo *root, Node *hashkey, double nbuckets)
 		if (get_attstatsslot(vardata.statsTuple,
 							 vardata.atttype, vardata.atttypmod,
 							 STATISTIC_KIND_MCV, InvalidOid,
-							 NULL, NULL, &numbers, &nnumbers))
+							 NULL,
+							 NULL, NULL,
+							 &numbers, &nnumbers))
 		{
 			/*
 			 * The first MCV stat is for the most common value.
@@ -4339,6 +4407,18 @@ get_variable_range(PlannerInfo *root, VariableStatData *vardata, Oid sortop,
 	int			nvalues;
 	int			i;
 
+	/*
+	 * XXX It's very tempting to try to use the actual column min and max,
+	 * if we can get them relatively-cheaply with an index probe.  However,
+	 * since this function is called many times during join planning,
+	 * that could have unpleasant effects on planning speed.  Need more
+	 * investigation before enabling this.
+	 */
+#ifdef NOT_USED
+	if (get_actual_variable_range(root, vardata, sortop, min, max))
+		return true;
+#endif
+
 	if (!HeapTupleIsValid(vardata->statsTuple))
 	{
 		/* no stats available, so default result */
@@ -4358,6 +4438,7 @@ get_variable_range(PlannerInfo *root, VariableStatData *vardata, Oid sortop,
 	if (get_attstatsslot(vardata->statsTuple,
 						 vardata->atttype, vardata->atttypmod,
 						 STATISTIC_KIND_HISTOGRAM, sortop,
+						 NULL,
 						 &values, &nvalues,
 						 NULL, NULL))
 	{
@@ -4372,6 +4453,7 @@ get_variable_range(PlannerInfo *root, VariableStatData *vardata, Oid sortop,
 	else if (get_attstatsslot(vardata->statsTuple,
 							  vardata->atttype, vardata->atttypmod,
 							  STATISTIC_KIND_HISTOGRAM, InvalidOid,
+							  NULL,
 							  &values, &nvalues,
 							  NULL, NULL))
 	{
@@ -4388,6 +4470,7 @@ get_variable_range(PlannerInfo *root, VariableStatData *vardata, Oid sortop,
 	if (get_attstatsslot(vardata->statsTuple,
 						 vardata->atttype, vardata->atttypmod,
 						 STATISTIC_KIND_MCV, InvalidOid,
+						 NULL,
 						 &values, &nvalues,
 						 NULL, NULL))
 	{
@@ -4429,6 +4512,205 @@ get_variable_range(PlannerInfo *root, VariableStatData *vardata, Oid sortop,
 }
 
 
+/*
+ * get_actual_variable_range
+ *		Attempt to identify the current *actual* minimum and/or maximum
+ *		of the specified variable, by looking for a suitable btree index
+ *		and fetching its low and/or high values.
+ *		If successful, store values in *min and *max, and return TRUE.
+ *		(Either pointer can be NULL if that endpoint isn't needed.)
+ *		If no data available, return FALSE.
+ *
+ * sortop is the "<" comparison operator to use.
+ */
+static bool
+get_actual_variable_range(PlannerInfo *root, VariableStatData *vardata,
+						  Oid sortop,
+						  Datum *min, Datum *max)
+{
+	bool		have_data = false;
+	RelOptInfo *rel = vardata->rel;
+	RangeTblEntry *rte;
+	ListCell   *lc;
+
+	/* No hope if no relation or it doesn't have indexes */
+	if (rel == NULL || rel->indexlist == NIL)
+		return false;
+	/* If it has indexes it must be a plain relation */
+	rte = root->simple_rte_array[rel->relid];
+	Assert(rte->rtekind == RTE_RELATION);
+
+	/* Search through the indexes to see if any match our problem */
+	foreach(lc, rel->indexlist)
+	{
+		IndexOptInfo *index = (IndexOptInfo *) lfirst(lc);
+		ScanDirection indexscandir;
+
+		/* Ignore non-btree indexes */
+		if (index->relam != BTREE_AM_OID)
+			continue;
+
+		/*
+		 * Ignore partial indexes --- we only want stats that cover the
+		 * entire relation.
+		 */
+		if (index->indpred != NIL)
+			continue;
+
+		/*
+		 * The index list might include fictitious indexes inserted by a
+		 * get_relation_info hook --- don't try to access them.
+		 */
+		if (!OidIsValid(index->indexoid))
+			continue;
+
+		/*
+		 * The first index column must match the desired variable and sort
+		 * operator --- but we can use a descending-order index.
+		 */
+		if (sortop == index->fwdsortop[0])
+			indexscandir = ForwardScanDirection;
+		else if (sortop == index->revsortop[0])
+			indexscandir = BackwardScanDirection;
+		else
+			continue;
+		if (!match_index_to_operand(vardata->var, 0, index))
+			continue;
+
+		/*
+		 * Found a suitable index to extract data from.  We'll need an
+		 * EState and a bunch of other infrastructure.
+		 */
+		{
+			EState	   *estate;
+			ExprContext *econtext;
+			MemoryContext tmpcontext;
+			MemoryContext oldcontext;
+			Relation	heapRel;
+			Relation	indexRel;
+			IndexInfo  *indexInfo;
+			TupleTableSlot *slot;
+			int16		typLen;
+			bool		typByVal;
+			ScanKeyData scankeys[1];
+			IndexScanDesc index_scan;
+			HeapTuple	tup;
+			Datum		values[INDEX_MAX_KEYS];
+			bool		isnull[INDEX_MAX_KEYS];
+
+			estate = CreateExecutorState();
+			econtext = GetPerTupleExprContext(estate);
+			/* Make sure any cruft is generated in the econtext's memory */
+			tmpcontext = econtext->ecxt_per_tuple_memory;
+			oldcontext = MemoryContextSwitchTo(tmpcontext);
+
+			/*
+			 * Open the table and index so we can read from them.  We should
+			 * already have at least AccessShareLock on the table, but not
+			 * necessarily on the index.
+			 */
+			heapRel = heap_open(rte->relid, NoLock);
+			indexRel = index_open(index->indexoid, AccessShareLock);
+
+			/* extract index key information from the index's pg_index info */
+			indexInfo = BuildIndexInfo(indexRel);
+
+			/* some other stuff */
+			slot = MakeSingleTupleTableSlot(RelationGetDescr(heapRel));
+			econtext->ecxt_scantuple = slot;
+			get_typlenbyval(vardata->atttype, &typLen, &typByVal);
+
+			/* set up an IS NOT NULL scan key so that we ignore nulls */
+			ScanKeyEntryInitialize(&scankeys[0],
+								   SK_ISNULL | SK_SEARCHNOTNULL,
+								   1,			/* index col to scan */
+								   InvalidStrategy,		/* no strategy */
+								   InvalidOid,	/* no strategy subtype */
+								   InvalidOid,	/* no reg proc for this */
+								   (Datum) 0);	/* constant */
+
+			have_data = true;
+
+			/* If min is requested ... */
+			if (min)
+			{
+				index_scan = index_beginscan(heapRel, indexRel, SnapshotNow,
+											 1, scankeys);
+
+				/* Fetch first tuple in sortop's direction */
+				if ((tup = index_getnext(index_scan,
+										 indexscandir)) != NULL)
+				{
+					/* Extract the index column values from the heap tuple */
+					ExecStoreTuple(tup,	slot, InvalidBuffer, false);
+					FormIndexDatum(indexInfo, slot, estate,
+								   values, isnull);
+
+					/* Shouldn't have got a null, but be careful */
+					if (isnull[0])
+						elog(ERROR, "found unexpected null value in index \"%s\"",
+							 RelationGetRelationName(indexRel));
+
+					/* Copy the index column value out to caller's context */
+					MemoryContextSwitchTo(oldcontext);
+					*min = datumCopy(values[0], typByVal, typLen);
+					MemoryContextSwitchTo(tmpcontext);
+				}
+				else
+					have_data = false;
+
+				index_endscan(index_scan);
+			}
+
+			/* If max is requested, and we didn't find the index is empty */
+			if (max && have_data)
+			{
+				index_scan = index_beginscan(heapRel, indexRel, SnapshotNow,
+											 1, scankeys);
+
+				/* Fetch first tuple in reverse direction */
+				if ((tup = index_getnext(index_scan,
+										 -indexscandir)) != NULL)
+				{
+					/* Extract the index column values from the heap tuple */
+					ExecStoreTuple(tup,	slot, InvalidBuffer, false);
+					FormIndexDatum(indexInfo, slot, estate,
+								   values, isnull);
+
+					/* Shouldn't have got a null, but be careful */
+					if (isnull[0])
+						elog(ERROR, "found unexpected null value in index \"%s\"",
+							 RelationGetRelationName(indexRel));
+
+					/* Copy the index column value out to caller's context */
+					MemoryContextSwitchTo(oldcontext);
+					*max = datumCopy(values[0], typByVal, typLen);
+					MemoryContextSwitchTo(tmpcontext);
+				}
+				else
+					have_data = false;
+
+				index_endscan(index_scan);
+			}
+
+			/* Clean everything up */
+			ExecDropSingleTupleTableSlot(slot);
+
+			index_close(indexRel, AccessShareLock);
+			heap_close(heapRel, NoLock);
+
+			MemoryContextSwitchTo(oldcontext);
+			FreeExecutorState(estate);
+
+			/* And we're done */
+			break;
+		}
+	}
+
+	return have_data;
+}
+
+
 /*-------------------------------------------------------------------------
  *
  * Pattern analysis functions
@@ -4795,7 +5077,7 @@ pattern_fixed_prefix(Const *patt, Pattern_Type ptype,
  * more useful to use the upper-bound code than not.
  */
 static Selectivity
-prefix_selectivity(VariableStatData *vardata,
+prefix_selectivity(PlannerInfo *root, VariableStatData *vardata,
 				   Oid vartype, Oid opfamily, Const *prefixcon)
 {
 	Selectivity prefixsel;
@@ -4810,11 +5092,11 @@ prefix_selectivity(VariableStatData *vardata,
 		elog(ERROR, "no >= operator for opfamily %u", opfamily);
 	fmgr_info(get_opcode(cmpopr), &opproc);
 
-	prefixsel = ineq_histogram_selectivity(vardata, &opproc, true,
+	prefixsel = ineq_histogram_selectivity(root, vardata, &opproc, true,
 										   prefixcon->constvalue,
 										   prefixcon->consttype);
 
-	if (prefixsel <= 0.0)
+	if (prefixsel < 0.0)
 	{
 		/* No histogram is present ... return a suitable default estimate */
 		return DEFAULT_MATCH_SEL;
@@ -4836,12 +5118,12 @@ prefix_selectivity(VariableStatData *vardata,
 	{
 		Selectivity topsel;
 
-		topsel = ineq_histogram_selectivity(vardata, &opproc, false,
+		topsel = ineq_histogram_selectivity(root, vardata, &opproc, false,
 											greaterstrcon->constvalue,
 											greaterstrcon->consttype);
 
 		/* ineq_histogram_selectivity worked before, it shouldn't fail now */
-		Assert(topsel > 0.0);
+		Assert(topsel >= 0.0);
 
 		/*
 		 * Merge the two selectivities in the same way as for a range query
@@ -5870,7 +6152,9 @@ btcostestimate(PG_FUNCTION_ARGS)
 		if (get_attstatsslot(vardata.statsTuple, InvalidOid, 0,
 							 STATISTIC_KIND_CORRELATION,
 							 index->fwdsortop[0],
-							 NULL, NULL, &numbers, &nnumbers))
+							 NULL,
+							 NULL, NULL,
+							 &numbers, &nnumbers))
 		{
 			double		varCorrelation;
 
@@ -5887,7 +6171,9 @@ btcostestimate(PG_FUNCTION_ARGS)
 		else if (get_attstatsslot(vardata.statsTuple, InvalidOid, 0,
 								  STATISTIC_KIND_CORRELATION,
 								  index->revsortop[0],
-								  NULL, NULL, &numbers, &nnumbers))
+								  NULL,
+								  NULL, NULL,
+								  &numbers, &nnumbers))
 		{
 			double		varCorrelation;
 

src/backend/utils/cache/lsyscache.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/cache/lsyscache.c,v 1.165 2010/01/02 16:57:55 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/cache/lsyscache.c,v 1.166 2010/01/04 02:44:40 tgl Exp $
  *
  * NOTES
  *	  Eventually, the index information should go through here, too.
@@ -2577,6 +2577,7 @@ get_attavgwidth(Oid relid, AttrNumber attnum)
  * atttypmod: typmod of attribute (can be 0 if values == NULL).
  * reqkind: STAKIND code for desired statistics slot kind.
  * reqop: STAOP value wanted, or InvalidOid if don't care.
+ * actualop: if not NULL, *actualop receives the actual STAOP value.
  * values, nvalues: if not NULL, the slot's stavalues are extracted.
  * numbers, nnumbers: if not NULL, the slot's stanumbers are extracted.
  *
@@ -2589,6 +2590,7 @@ bool
 get_attstatsslot(HeapTuple statstuple,
 				 Oid atttype, int32 atttypmod,
 				 int reqkind, Oid reqop,
+				 Oid *actualop,
 				 Datum **values, int *nvalues,
 				 float4 **numbers, int *nnumbers)
 {
@@ -2611,6 +2613,9 @@ get_attstatsslot(HeapTuple statstuple,
 	if (i >= STATISTIC_NUM_SLOTS)
 		return false;			/* not there */
 
+	if (actualop)
+		*actualop = (&stats->staop1)[i];
+
 	if (values)
 	{
 		val = SysCacheGetAttr(STATRELATTINH, statstuple,

src/include/utils/lsyscache.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2010, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/lsyscache.h,v 1.130 2010/01/02 16:58:10 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/utils/lsyscache.h,v 1.131 2010/01/04 02:44:40 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -129,6 +129,7 @@ extern int32 get_attavgwidth(Oid relid, AttrNumber attnum);
 extern bool get_attstatsslot(HeapTuple statstuple,
 				 Oid atttype, int32 atttypmod,
 				 int reqkind, Oid reqop,
+				 Oid *actualop,
 				 Datum **values, int *nvalues,
 				 float4 **numbers, int *nnumbers);
 extern void free_attstatsslot(Oid atttype,