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

The mess cleaned up in commit da0759600 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

1 files changed, 71 insertions, 78 deletions

diff --git a/src/backend/utils/cache/lsyscache.c b/src/backend/utils/cache/lsyscache.c
index 720e5402768..b94d4755055 100644
--- a/src/backend/utils/cache/lsyscache.c
+++ b/src/backend/utils/cache/lsyscache.c
@@ -32,7 +32,6 @@
 #include "catalog/pg_proc.h"
 #include "catalog/pg_range.h"
 #include "catalog/pg_statistic.h"
-#include "catalog/pg_statistic_ext.h"
 #include "catalog/pg_transform.h"
 #include "catalog/pg_type.h"
 #include "miscadmin.h"
@@ -2865,35 +2864,39 @@ get_attavgwidth(Oid relid, AttrNumber attnum)
  * that have been provided by a stats hook and didn't really come from
  * pg_statistic.
  *
+ * sslot: pointer to output area (typically, a local variable in the caller).
  * statstuple: pg_statistic tuple to be examined.
- * atttype: type OID of slot's stavalues (can be InvalidOid if values == NULL).
- * atttypmod: typmod of slot's stavalues (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.
+ * flags: bitmask of ATTSTATSSLOT_VALUES and/or ATTSTATSSLOT_NUMBERS.
  *
- * If assigned, values and numbers are set to point to palloc'd arrays.
- * If the stavalues datatype is pass-by-reference, the values referenced by
- * the values array are themselves palloc'd.  The palloc'd stuff can be
- * freed by calling free_attstatsslot.
+ * If a matching slot is found, TRUE is returned, and *sslot is filled thus:
+ * staop: receives the actual STAOP value.
+ * valuetype: receives actual datatype of the elements of stavalues.
+ * values: receives pointer to an array of the slot's stavalues.
+ * nvalues: receives number of stavalues.
+ * numbers: receives pointer to an array of the slot's stanumbers (as float4).
+ * nnumbers: receives number of stanumbers.
  *
- * Note: at present, atttype/atttypmod aren't actually used here at all.
- * But the caller must have the correct (or at least binary-compatible)
- * type ID to pass to free_attstatsslot later.
+ * valuetype/values/nvalues are InvalidOid/NULL/0 if ATTSTATSSLOT_VALUES
+ * wasn't specified.  Likewise, numbers/nnumbers are NULL/0 if
+ * ATTSTATSSLOT_NUMBERS wasn't specified.
+ *
+ * If no matching slot is found, FALSE is returned, and *sslot is zeroed.
+ *
+ * The data referred to by the fields of sslot is locally palloc'd and
+ * is independent of the original pg_statistic tuple.  When the caller
+ * is done with it, call free_attstatsslot to release the palloc'd data.
+ *
+ * If it's desirable to call free_attstatsslot when get_attstatsslot might
+ * not have been called, memset'ing sslot to zeroes will allow that.
  */
 bool
-get_attstatsslot(HeapTuple statstuple,
-				 Oid atttype, int32 atttypmod,
-				 int reqkind, Oid reqop,
-				 Oid *actualop,
-				 Datum **values, int *nvalues,
-				 float4 **numbers, int *nnumbers)
+get_attstatsslot(AttStatsSlot *sslot, HeapTuple statstuple,
+				 int reqkind, Oid reqop, int flags)
 {
 	Form_pg_statistic stats = (Form_pg_statistic) GETSTRUCT(statstuple);
-	int			i,
-				j;
+	int			i;
 	Datum		val;
 	bool		isnull;
 	ArrayType  *statarray;
@@ -2902,6 +2905,9 @@ get_attstatsslot(HeapTuple statstuple,
 	HeapTuple	typeTuple;
 	Form_pg_type typeForm;
 
+	/* initialize *sslot properly */
+	memset(sslot, 0, sizeof(AttStatsSlot));
+
 	for (i = 0; i < STATISTIC_NUM_SLOTS; i++)
 	{
 		if ((&stats->stakind1)[i] == reqkind &&
@@ -2911,26 +2917,29 @@ get_attstatsslot(HeapTuple statstuple,
 	if (i >= STATISTIC_NUM_SLOTS)
 		return false;			/* not there */
 
-	if (actualop)
-		*actualop = (&stats->staop1)[i];
+	sslot->staop = (&stats->staop1)[i];
 
-	if (values)
+	if (flags & ATTSTATSSLOT_VALUES)
 	{
 		val = SysCacheGetAttr(STATRELATTINH, statstuple,
 							  Anum_pg_statistic_stavalues1 + i,
 							  &isnull);
 		if (isnull)
 			elog(ERROR, "stavalues is null");
-		statarray = DatumGetArrayTypeP(val);
 
 		/*
-		 * Need to get info about the array element type.  We look at the
-		 * actual element type embedded in the array, which might be only
-		 * binary-compatible with the passed-in atttype.  The info we extract
-		 * here should be the same either way, but deconstruct_array is picky
-		 * about having an exact type OID match.
+		 * Detoast the array if needed, and in any case make a copy that's
+		 * under control of this AttStatsSlot.
 		 */
-		arrayelemtype = ARR_ELEMTYPE(statarray);
+		statarray = DatumGetArrayTypePCopy(val);
+
+		/*
+		 * Extract the actual array element type, and pass it back in case the
+		 * caller needs it.
+		 */
+		sslot->valuetype = arrayelemtype = ARR_ELEMTYPE(statarray);
+
+		/* Need info about element type */
 		typeTuple = SearchSysCache1(TYPEOID, ObjectIdGetDatum(arrayelemtype));
 		if (!HeapTupleIsValid(typeTuple))
 			elog(ERROR, "cache lookup failed for type %u", arrayelemtype);
@@ -2942,40 +2951,35 @@ get_attstatsslot(HeapTuple statstuple,
 						  typeForm->typlen,
 						  typeForm->typbyval,
 						  typeForm->typalign,
-						  values, NULL, nvalues);
+						  &sslot->values, NULL, &sslot->nvalues);
 
 		/*
 		 * If the element type is pass-by-reference, we now have a bunch of
-		 * Datums that are pointers into the syscache value.  Copy them to
-		 * avoid problems if syscache decides to drop the entry.
+		 * Datums that are pointers into the statarray, so we need to keep
+		 * that until free_attstatsslot.  Otherwise, all the useful info is in
+		 * sslot->values[], so we can free the array object immediately.
 		 */
 		if (!typeForm->typbyval)
-		{
-			for (j = 0; j < *nvalues; j++)
-			{
-				(*values)[j] = datumCopy((*values)[j],
-										 typeForm->typbyval,
-										 typeForm->typlen);
-			}
-		}
+			sslot->values_arr = statarray;
+		else
+			pfree(statarray);
 
 		ReleaseSysCache(typeTuple);
-
-		/*
-		 * Free statarray if it's a detoasted copy.
-		 */
-		if ((Pointer) statarray != DatumGetPointer(val))
-			pfree(statarray);
 	}
 
-	if (numbers)
+	if (flags & ATTSTATSSLOT_NUMBERS)
 	{
 		val = SysCacheGetAttr(STATRELATTINH, statstuple,
 							  Anum_pg_statistic_stanumbers1 + i,
 							  &isnull);
 		if (isnull)
 			elog(ERROR, "stanumbers is null");
-		statarray = DatumGetArrayTypeP(val);
+
+		/*
+		 * Detoast the array if needed, and in any case make a copy that's
+		 * under control of this AttStatsSlot.
+		 */
+		statarray = DatumGetArrayTypePCopy(val);
 
 		/*
 		 * We expect the array to be a 1-D float4 array; verify that. We don't
@@ -2987,15 +2991,13 @@ get_attstatsslot(HeapTuple statstuple,
 			ARR_HASNULL(statarray) ||
 			ARR_ELEMTYPE(statarray) != FLOAT4OID)
 			elog(ERROR, "stanumbers is not a 1-D float4 array");
-		*numbers = (float4 *) palloc(narrayelem * sizeof(float4));
-		memcpy(*numbers, ARR_DATA_PTR(statarray), narrayelem * sizeof(float4));
-		*nnumbers = narrayelem;
 
-		/*
-		 * Free statarray if it's a detoasted copy.
-		 */
-		if ((Pointer) statarray != DatumGetPointer(val))
-			pfree(statarray);
+		/* Give caller a pointer directly into the statarray */
+		sslot->numbers = (float4 *) ARR_DATA_PTR(statarray);
+		sslot->nnumbers = narrayelem;
+
+		/* We'll free the statarray in free_attstatsslot */
+		sslot->numbers_arr = statarray;
 	}
 
 	return true;
@@ -3004,28 +3006,19 @@ get_attstatsslot(HeapTuple statstuple,
 /*
  * free_attstatsslot
  *		Free data allocated by get_attstatsslot
- *
- * atttype is the type of the individual values in values[].
- * It need be valid only if values != NULL.
  */
 void
-free_attstatsslot(Oid atttype,
-				  Datum *values, int nvalues,
-				  float4 *numbers, int nnumbers)
-{
-	if (values)
-	{
-		if (!get_typbyval(atttype))
-		{
-			int			i;
-
-			for (i = 0; i < nvalues; i++)
-				pfree(DatumGetPointer(values[i]));
-		}
-		pfree(values);
-	}
-	if (numbers)
-		pfree(numbers);
+free_attstatsslot(AttStatsSlot *sslot)
+{
+	/* The values[] array was separately palloc'd by deconstruct_array */
+	if (sslot->values)
+		pfree(sslot->values);
+	/* The numbers[] array points into numbers_arr, do not pfree it */
+	/* Free the detoasted array objects, if any */
+	if (sslot->values_arr)
+		pfree(sslot->values_arr);
+	if (sslot->numbers_arr)
+		pfree(sslot->numbers_arr);
 }
 
 /*				---------- PG_NAMESPACE CACHE ----------				 */