Redesign get_attstatsslot()/free_attstatsslot() for more safety and speed.

The mess cleaned up in commit da07596 is clear evidence that it's a
bug hazard to expect the caller of get_attstatsslot()/free_attstatsslot()
to provide the correct type OID for the array elements in the slot.
Moreover, we weren't even getting any performance benefit from that,
since get_attstatsslot() was extracting the real type OID from the array
anyway.  So we ought to get rid of that requirement; indeed, it would
make more sense for get_attstatsslot() to pass back the type OID it found,
in case the caller isn't sure what to expect, which is likely in binary-
compatible-operator cases.

Another problem with the current implementation is that if the stats array
element type is pass-by-reference, we incur a palloc/memcpy/pfree cycle
for each element.  That seemed acceptable when the code was written because
we were targeting O(10) array sizes --- but these days, stats arrays are
almost always bigger than that, sometimes much bigger.  We can save a
significant number of cycles by doing one palloc/memcpy/pfree of the whole
array.  Indeed, in the now-probably-common case where the array is toasted,
that happens anyway so this method is basically free.  (Note: although the
catcache code will inline any out-of-line toasted values, it doesn't
decompress them.  At the other end of the size range, it doesn't expand
short-header datums either.  In either case, DatumGetArrayTypeP would have
to make a copy.  We do end up using an extra array copy step if the element
type is pass-by-value and the array length is neither small enough for a
short header nor large enough to have suffered compression.  But that
seems like a very acceptable price for winning in pass-by-ref cases.)

Hence, redesign to take these insights into account.  While at it,
convert to an API in which we fill a struct rather than passing a bunch
of pointers to individual output arguments.  That will make it less
painful if we ever want further expansion of what get_attstatsslot can
pass back.

It's certainly arguable that this is new development and not something to
push post-feature-freeze.  However, I view it as primarily bug-proofing
and therefore something that's better to have sooner not later.  Since
we aren't quite at beta phase yet, let's put it in.

Discussion: https://postgr.es/m/16364.1494520862@sss.pgh.pa.us

Author: tglsfdc

contrib/intarray/_int_selfuncs.c

@@ -136,11 +136,7 @@ _int_matchsel(PG_FUNCTION_ARGS)
 	int			nmcelems = 0;
 	float4		minfreq = 0.0;
 	float4		nullfrac = 0.0;
-	Form_pg_statistic stats;
-	Datum	   *values = NULL;
-	int			nvalues = 0;
-	float4	   *numbers = NULL;
-	int			nnumbers = 0;
+	AttStatsSlot sslot;
 
 	/*
 	 * If expression is not "variable @@ something" or "something @@ variable"
@@ -193,36 +189,39 @@ _int_matchsel(PG_FUNCTION_ARGS)
 	 */
 	if (HeapTupleIsValid(vardata.statsTuple))
 	{
+		Form_pg_statistic stats;
+
 		stats = (Form_pg_statistic) GETSTRUCT(vardata.statsTuple);
 		nullfrac = stats->stanullfrac;
 
 		/*
 		 * For an int4 array, the default array type analyze function will
 		 * collect a Most Common Elements list, which is an array of int4s.
 		 */
-		if (get_attstatsslot(vardata.statsTuple,
-							 INT4OID, -1,
+		if (get_attstatsslot(&sslot, vardata.statsTuple,
 							 STATISTIC_KIND_MCELEM, InvalidOid,
-							 NULL,
-							 &values, &nvalues,
-							 &numbers, &nnumbers))
+							 ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS))
 		{
+			Assert(sslot.valuetype == INT4OID);
+
 			/*
 			 * There should be three more Numbers than Values, because the
 			 * last three (for intarray) cells are taken for minimal, maximal
 			 * and nulls frequency. Punt if not.
 			 */
-			if (nnumbers == nvalues + 3)
+			if (sslot.nnumbers == sslot.nvalues + 3)
 			{
 				/* Grab the lowest frequency. */
-				minfreq = numbers[nnumbers - (nnumbers - nvalues)];
+				minfreq = sslot.numbers[sslot.nnumbers - (sslot.nnumbers - sslot.nvalues)];
 
-				mcelems = values;
-				mcefreqs = numbers;
-				nmcelems = nvalues;
+				mcelems = sslot.values;
+				mcefreqs = sslot.numbers;
+				nmcelems = sslot.nvalues;
 			}
 		}
 	}
+	else
+		memset(&sslot, 0, sizeof(sslot));
 
 	/* Process the logical expression in the query, using the stats */
 	selec = int_query_opr_selec(GETQUERY(query) + query->size - 1,
@@ -231,7 +230,7 @@ _int_matchsel(PG_FUNCTION_ARGS)
 	/* MCE stats count only non-null rows, so adjust for null rows. */
 	selec *= (1.0 - nullfrac);
 
-	free_attstatsslot(INT4OID, values, nvalues, numbers, nnumbers);
+	free_attstatsslot(&sslot);
 	ReleaseVariableStats(vardata);
 
 	CLAMP_PROBABILITY(selec);

src/backend/executor/nodeHash.c

@@ -1283,10 +1283,7 @@ static void
 ExecHashBuildSkewHash(HashJoinTable hashtable, Hash *node, int mcvsToUse)
 {
 	HeapTupleData *statsTuple;
-	Datum	   *values;
-	int			nvalues;
-	float4	   *numbers;
-	int			nnumbers;
+	AttStatsSlot sslot;
 
 	/* Do nothing if planner didn't identify the outer relation's join key */
 	if (!OidIsValid(node->skewTable))
@@ -1305,19 +1302,17 @@ ExecHashBuildSkewHash(HashJoinTable hashtable, Hash *node, int mcvsToUse)
 	if (!HeapTupleIsValid(statsTuple))
 		return;
 
-	if (get_attstatsslot(statsTuple, node->skewColType, node->skewColTypmod,
+	if (get_attstatsslot(&sslot, statsTuple,
 						 STATISTIC_KIND_MCV, InvalidOid,
-						 NULL,
-						 &values, &nvalues,
-						 &numbers, &nnumbers))
+						 ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS))
 	{
 		double		frac;
 		int			nbuckets;
 		FmgrInfo   *hashfunctions;
 		int			i;
 
-		if (mcvsToUse > nvalues)
-			mcvsToUse = nvalues;
+		if (mcvsToUse > sslot.nvalues)
+			mcvsToUse = sslot.nvalues;
 
 		/*
 		 * Calculate the expected fraction of outer relation that will
@@ -1326,11 +1321,10 @@ ExecHashBuildSkewHash(HashJoinTable hashtable, Hash *node, int mcvsToUse)
 		 */
 		frac = 0;
 		for (i = 0; i < mcvsToUse; i++)
-			frac += numbers[i];
+			frac += sslot.numbers[i];
 		if (frac < SKEW_MIN_OUTER_FRACTION)
 		{
-			free_attstatsslot(node->skewColType,
-							  values, nvalues, numbers, nnumbers);
+			free_attstatsslot(&sslot);
 			ReleaseSysCache(statsTuple);
 			return;
 		}
@@ -1392,7 +1386,7 @@ ExecHashBuildSkewHash(HashJoinTable hashtable, Hash *node, int mcvsToUse)
 			int			bucket;
 
 			hashvalue = DatumGetUInt32(FunctionCall1(&hashfunctions[0],
-													 values[i]));
+													 sslot.values[i]));
 
 			/*
 			 * While we have not hit a hole in the hashtable and have not hit
@@ -1426,8 +1420,7 @@ ExecHashBuildSkewHash(HashJoinTable hashtable, Hash *node, int mcvsToUse)
 				hashtable->spacePeak = hashtable->spaceUsed;
 		}
 
-		free_attstatsslot(node->skewColType,
-						  values, nvalues, numbers, nnumbers);
+		free_attstatsslot(&sslot);
 	}
 
 	ReleaseSysCache(statsTuple);

src/backend/tsearch/ts_selfuncs.c

@@ -163,28 +163,22 @@ tsquerysel(VariableStatData *vardata, Datum constval)
 	if (HeapTupleIsValid(vardata->statsTuple))
 	{
 		Form_pg_statistic stats;
-		Datum	   *values;
-		int			nvalues;
-		float4	   *numbers;
-		int			nnumbers;
+		AttStatsSlot sslot;
 
 		stats = (Form_pg_statistic) GETSTRUCT(vardata->statsTuple);
 
 		/* MCELEM will be an array of TEXT elements for a tsvector column */
-		if (get_attstatsslot(vardata->statsTuple,
-							 TEXTOID, -1,
+		if (get_attstatsslot(&sslot, vardata->statsTuple,
 							 STATISTIC_KIND_MCELEM, InvalidOid,
-							 NULL,
-							 &values, &nvalues,
-							 &numbers, &nnumbers))
+							 ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS))
 		{
 			/*
 			 * There is a most-common-elements slot for the tsvector Var, so
 			 * use that.
 			 */
-			selec = mcelem_tsquery_selec(query, values, nvalues,
-										 numbers, nnumbers);
-			free_attstatsslot(TEXTOID, values, nvalues, numbers, nnumbers);
+			selec = mcelem_tsquery_selec(query, sslot.values, sslot.nvalues,
+										 sslot.numbers, sslot.nnumbers);
+			free_attstatsslot(&sslot);
 		}
 		else
 		{

src/backend/utils/adt/array_selfuncs.c

@@ -137,58 +137,49 @@ scalararraysel_containment(PlannerInfo *root,
 		statistic_proc_security_check(&vardata, cmpfunc->fn_oid))
 	{
 		Form_pg_statistic stats;
-		Datum	   *values;
-		int			nvalues;
-		float4	   *numbers;
-		int			nnumbers;
-		float4	   *hist;
-		int			nhist;
+		AttStatsSlot sslot;
+		AttStatsSlot hslot;
 
 		stats = (Form_pg_statistic) GETSTRUCT(vardata.statsTuple);
 
 		/* MCELEM will be an array of same type as element */
-		if (get_attstatsslot(vardata.statsTuple,
-							 elemtype, vardata.atttypmod,
+		if (get_attstatsslot(&sslot, vardata.statsTuple,
 							 STATISTIC_KIND_MCELEM, InvalidOid,
-							 NULL,
-							 &values, &nvalues,
-							 &numbers, &nnumbers))
+							 ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS))
 		{
 			/* For ALL case, also get histogram of distinct-element counts */
 			if (useOr ||
-				!get_attstatsslot(vardata.statsTuple,
-								  elemtype, vardata.atttypmod,
+				!get_attstatsslot(&hslot, vardata.statsTuple,
 								  STATISTIC_KIND_DECHIST, InvalidOid,
-								  NULL,
-								  NULL, NULL,
-								  &hist, &nhist))
-			{
-				hist = NULL;
-				nhist = 0;
-			}
+								  ATTSTATSSLOT_NUMBERS))
+				memset(&hslot, 0, sizeof(hslot));
 
 			/*
 			 * For = ANY, estimate as var @> ARRAY[const].
 			 *
 			 * For = ALL, estimate as var <@ ARRAY[const].
 			 */
 			if (useOr)
-				selec = mcelem_array_contain_overlap_selec(values, nvalues,
-														   numbers, nnumbers,
+				selec = mcelem_array_contain_overlap_selec(sslot.values,
+														   sslot.nvalues,
+														   sslot.numbers,
+														   sslot.nnumbers,
 														   &constval, 1,
 													   OID_ARRAY_CONTAINS_OP,
 														   cmpfunc);
 			else
-				selec = mcelem_array_contained_selec(values, nvalues,
-													 numbers, nnumbers,
+				selec = mcelem_array_contained_selec(sslot.values,
+													 sslot.nvalues,
+													 sslot.numbers,
+													 sslot.nnumbers,
 													 &constval, 1,
-													 hist, nhist,
+													 hslot.numbers,
+													 hslot.nnumbers,
 													 OID_ARRAY_CONTAINED_OP,
 													 cmpfunc);
 
-			if (hist)
-				free_attstatsslot(elemtype, NULL, 0, hist, nhist);
-			free_attstatsslot(elemtype, values, nvalues, numbers, nnumbers);
+			free_attstatsslot(&hslot);
+			free_attstatsslot(&sslot);
 		}
 		else
 		{
@@ -369,49 +360,35 @@ calc_arraycontsel(VariableStatData *vardata, Datum constval,
 		statistic_proc_security_check(vardata, cmpfunc->fn_oid))
 	{
 		Form_pg_statistic stats;
-		Datum	   *values;
-		int			nvalues;
-		float4	   *numbers;
-		int			nnumbers;
-		float4	   *hist;
-		int			nhist;
+		AttStatsSlot sslot;
+		AttStatsSlot hslot;
 
 		stats = (Form_pg_statistic) GETSTRUCT(vardata->statsTuple);
 
 		/* MCELEM will be an array of same type as column */
-		if (get_attstatsslot(vardata->statsTuple,
-							 elemtype, vardata->atttypmod,
+		if (get_attstatsslot(&sslot, vardata->statsTuple,
 							 STATISTIC_KIND_MCELEM, InvalidOid,
-							 NULL,
-							 &values, &nvalues,
-							 &numbers, &nnumbers))
+							 ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS))
 		{
 			/*
 			 * For "array <@ const" case we also need histogram of distinct
 			 * element counts.
 			 */
 			if (operator != OID_ARRAY_CONTAINED_OP ||
-				!get_attstatsslot(vardata->statsTuple,
-								  elemtype, vardata->atttypmod,
+				!get_attstatsslot(&hslot, vardata->statsTuple,
 								  STATISTIC_KIND_DECHIST, InvalidOid,
-								  NULL,
-								  NULL, NULL,
-								  &hist, &nhist))
-			{
-				hist = NULL;
-				nhist = 0;
-			}
+								  ATTSTATSSLOT_NUMBERS))
+				memset(&hslot, 0, sizeof(hslot));
 
 			/* Use the most-common-elements slot for the array Var. */
 			selec = mcelem_array_selec(array, typentry,
-									   values, nvalues,
-									   numbers, nnumbers,
-									   hist, nhist,
+									   sslot.values, sslot.nvalues,
+									   sslot.numbers, sslot.nnumbers,
+									   hslot.numbers, hslot.nnumbers,
 									   operator, cmpfunc);
 
-			if (hist)
-				free_attstatsslot(elemtype, NULL, 0, hist, nhist);
-			free_attstatsslot(elemtype, values, nvalues, numbers, nnumbers);
+			free_attstatsslot(&hslot);
+			free_attstatsslot(&sslot);
 		}
 		else
 		{