Improve handling of NULLs in KNN-GiST and KNN-SP-GiST

This commit improves subject in two ways:

 * It removes ugliness of 02f9087, which stores distance values and null
   flags in two separate arrays after GISTSearchItem struct.  Instead we pack
   both distance value and null flag in IndexOrderByDistance struct.  Alignment
   overhead should be negligible, because we typically deal with at most few
   "col op const" expressions in ORDER BY clause.
 * It fixes handling of "col op NULL" expression in KNN-SP-GiST.  Now, these
   expression are not passed to support functions, which can't deal with them.
   Instead, NULL result is implicitly assumed.  It future we may decide to
   teach support functions to deal with NULL arguments, but current solution is
   bugfix suitable for backpatch.

Reported-by: Nikita Glukhov
Discussion: https://postgr.es/m/826f57ee-afc7-8977-c44c-6111d18b02ec%40postgrespro.ru
Author: Nikita Glukhov
Reviewed-by: Alexander Korotkov
Backpatch-through: 9.4

Author: akorotkov

src/backend/access/gist/gistget.c

@@ -112,9 +112,8 @@ gistkillitems(IndexScanDesc scan)
  * Similarly, *recheck_distances_p is set to indicate whether the distances
  * need to be rechecked, and it is also ignored for non-leaf entries.
  *
- * If we are doing an ordered scan, so->distancesValues[] and
- * so->distancesNulls[] is filled with distance data from the distance()
- * functions before returning success.
+ * If we are doing an ordered scan, so->distances[] is filled with distance
+ * data from the distance() functions before returning success.
  *
  * We must decompress the key in the IndexTuple before passing it to the
  * sk_funcs (which actually are the opclass Consistent or Distance methods).
@@ -135,8 +134,7 @@ gistindex_keytest(IndexScanDesc scan,
 	GISTSTATE  *giststate = so->giststate;
 	ScanKey		key = scan->keyData;
 	int			keySize = scan->numberOfKeys;
-	double	   *distance_value_p;
-	bool	   *distance_null_p;
+	IndexOrderByDistance *distance_p;
 	Relation	r = scan->indexRelation;
 
 	*recheck_p = false;
@@ -155,8 +153,8 @@ gistindex_keytest(IndexScanDesc scan,
 			elog(ERROR, "invalid GiST tuple found on leaf page");
 		for (i = 0; i < scan->numberOfOrderBys; i++)
 		{
-			so->distanceValues[i] = -get_float8_infinity();
-			so->distanceNulls[i] = false;
+			so->distances[i].value = -get_float8_infinity();
+			so->distances[i].isnull = false;
 		}
 		return true;
 	}
@@ -240,8 +238,7 @@ gistindex_keytest(IndexScanDesc scan,
 
 	/* OK, it passes --- now let's compute the distances */
 	key = scan->orderByData;
-	distance_value_p = so->distanceValues;
-	distance_null_p = so->distanceNulls;
+	distance_p = so->distances;
 	keySize = scan->numberOfOrderBys;
 	while (keySize > 0)
 	{
@@ -256,8 +253,8 @@ gistindex_keytest(IndexScanDesc scan,
 		if ((key->sk_flags & SK_ISNULL) || isNull)
 		{
 			/* Assume distance computes as null */
-			*distance_value_p = 0.0;
-			*distance_null_p = true;
+			distance_p->value = 0.0;
+			distance_p->isnull = true;
 		}
 		else
 		{
@@ -294,13 +291,12 @@ gistindex_keytest(IndexScanDesc scan,
 									 ObjectIdGetDatum(key->sk_subtype),
 									 PointerGetDatum(&recheck));
 			*recheck_distances_p |= recheck;
-			*distance_value_p = DatumGetFloat8(dist);
-			*distance_null_p = false;
+			distance_p->value = DatumGetFloat8(dist);
+			distance_p->isnull = false;
 		}
 
 		key++;
-		distance_value_p++;
-		distance_null_p++;
+		distance_p++;
 		keySize--;
 	}
 
@@ -313,8 +309,7 @@ gistindex_keytest(IndexScanDesc scan,
  *
  * scan: index scan we are executing
  * pageItem: search queue item identifying an index page to scan
- * myDistanceValues: distances array associated with pageItem, or NULL at the root
- * myDistanceNulls: null flags for myDistanceValues array, or NULL at the root
+ * myDistances: distances array associated with pageItem, or NULL at the root
  * tbm: if not NULL, gistgetbitmap's output bitmap
  * ntids: if not NULL, gistgetbitmap's output tuple counter
  *
@@ -332,8 +327,7 @@ gistindex_keytest(IndexScanDesc scan,
  */
 static void
 gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem,
-			 double *myDistanceValues, bool *myDistanceNulls,
-			 TIDBitmap *tbm, int64 *ntids)
+			 IndexOrderByDistance *myDistances, TIDBitmap *tbm, int64 *ntids)
 {
 	GISTScanOpaque so = (GISTScanOpaque) scan->opaque;
 	GISTSTATE  *giststate = so->giststate;
@@ -370,7 +364,7 @@ gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem,
 		GISTSearchItem *item;
 
 		/* This can't happen when starting at the root */
-		Assert(myDistanceValues != NULL && myDistanceNulls != NULL);
+		Assert(myDistances != NULL);
 
 		oldcxt = MemoryContextSwitchTo(so->queueCxt);
 
@@ -380,10 +374,8 @@ gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem,
 		item->data.parentlsn = pageItem->data.parentlsn;
 
 		/* Insert it into the queue using same distances as for this page */
-		memcpy(GISTSearchItemDistanceValues(item, scan->numberOfOrderBys),
-			   myDistanceValues, sizeof(double) * scan->numberOfOrderBys);
-		memcpy(GISTSearchItemDistanceNulls(item, scan->numberOfOrderBys),
-			   myDistanceNulls, sizeof(bool) * scan->numberOfOrderBys);
+		memcpy(item->distances, myDistances,
+			   sizeof(item->distances[0]) * scan->numberOfOrderBys);
 
 		pairingheap_add(so->queue, &item->phNode);
 
@@ -527,10 +519,8 @@ gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem,
 			}
 
 			/* Insert it into the queue using new distance data */
-			memcpy(GISTSearchItemDistanceValues(item, nOrderBys),
-				   so->distanceValues, sizeof(double) * nOrderBys);
-			memcpy(GISTSearchItemDistanceNulls(item, nOrderBys),
-				   so->distanceNulls, sizeof(bool) * nOrderBys);
+			memcpy(item->distances, so->distances,
+				   sizeof(item->distances[0]) * nOrderBys);
 
 			pairingheap_add(so->queue, &item->phNode);
 
@@ -595,8 +585,7 @@ getNextNearest(IndexScanDesc scan)
 			scan->xs_recheck = item->data.heap.recheck;
 
 			index_store_float8_orderby_distances(scan, so->orderByTypes,
-												 GISTSearchItemDistanceValues(item, scan->numberOfOrderBys),
-												 GISTSearchItemDistanceNulls(item, scan->numberOfOrderBys),
+												 item->distances,
 												 item->data.heap.recheckDistances);
 
 			/* in an index-only scan, also return the reconstructed tuple. */
@@ -609,10 +598,7 @@ getNextNearest(IndexScanDesc scan)
 			/* visit an index page, extract its items into queue */
 			CHECK_FOR_INTERRUPTS();
 
-			gistScanPage(scan, item,
-						 GISTSearchItemDistanceValues(item, scan->numberOfOrderBys),
-						 GISTSearchItemDistanceNulls(item, scan->numberOfOrderBys),
-						 NULL, NULL);
+			gistScanPage(scan, item, item->distances, NULL, NULL);
 		}
 
 		pfree(item);
@@ -650,7 +636,7 @@ gistgettuple(IndexScanDesc scan, ScanDirection dir)
 
 		fakeItem.blkno = GIST_ROOT_BLKNO;
 		memset(&fakeItem.data.parentlsn, 0, sizeof(GistNSN));
-		gistScanPage(scan, &fakeItem, NULL, NULL, NULL, NULL);
+		gistScanPage(scan, &fakeItem, NULL, NULL, NULL);
 	}
 
 	if (scan->numberOfOrderBys > 0)
@@ -744,10 +730,7 @@ gistgettuple(IndexScanDesc scan, ScanDirection dir)
 				 * this page, we fall out of the inner "do" and loop around to
 				 * return them.
 				 */
-				gistScanPage(scan, item,
-							 GISTSearchItemDistanceValues(item, scan->numberOfOrderBys),
-							 GISTSearchItemDistanceNulls(item, scan->numberOfOrderBys),
-							 NULL, NULL);
+				gistScanPage(scan, item, item->distances, NULL, NULL);
 
 				pfree(item);
 			} while (so->nPageData == 0);
@@ -778,7 +761,7 @@ gistgetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
 
 	fakeItem.blkno = GIST_ROOT_BLKNO;
 	memset(&fakeItem.data.parentlsn, 0, sizeof(GistNSN));
-	gistScanPage(scan, &fakeItem, NULL, NULL, tbm, &ntids);
+	gistScanPage(scan, &fakeItem, NULL, tbm, &ntids);
 
 	/*
 	 * While scanning a leaf page, ItemPointers of matching heap tuples will
@@ -793,10 +776,7 @@ gistgetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
 
 		CHECK_FOR_INTERRUPTS();
 
-		gistScanPage(scan, item,
-					 GISTSearchItemDistanceValues(item, scan->numberOfOrderBys),
-					 GISTSearchItemDistanceNulls(item, scan->numberOfOrderBys),
-					 tbm, &ntids);
+		gistScanPage(scan, item, item->distances, tbm, &ntids);
 
 		pfree(item);
 	}

src/backend/access/gist/gistscan.c

@@ -33,26 +33,23 @@ pairingheap_GISTSearchItem_cmp(const pairingheap_node *a, const pairingheap_node
 	const GISTSearchItem *sb = (const GISTSearchItem *) b;
 	IndexScanDesc scan = (IndexScanDesc) arg;
 	int			i;
-	double	   *da = GISTSearchItemDistanceValues(sa, scan->numberOfOrderBys),
-			   *db = GISTSearchItemDistanceValues(sb, scan->numberOfOrderBys);
-	bool	   *na = GISTSearchItemDistanceNulls(sa, scan->numberOfOrderBys),
-			   *nb = GISTSearchItemDistanceNulls(sb, scan->numberOfOrderBys);
 
 	/* Order according to distance comparison */
 	for (i = 0; i < scan->numberOfOrderBys; i++)
 	{
-		if (na[i])
+		if (sa->distances[i].isnull)
 		{
-			if (!nb[i])
+			if (!sb->distances[i].isnull)
 				return -1;
 		}
-		else if (nb[i])
+		else if (sb->distances[i].isnull)
 		{
 			return 1;
 		}
 		else
 		{
-			int			cmp = -float8_cmp_internal(da[i], db[i]);
+			int			cmp = -float8_cmp_internal(sa->distances[i].value,
+												   sb->distances[i].value);
 
 			if (cmp != 0)
 				return cmp;
@@ -100,8 +97,7 @@ gistbeginscan(Relation r, int nkeys, int norderbys)
 	so->queueCxt = giststate->scanCxt;	/* see gistrescan */
 
 	/* workspaces with size dependent on numberOfOrderBys: */
-	so->distanceValues = palloc(sizeof(double) * scan->numberOfOrderBys);
-	so->distanceNulls = palloc(sizeof(bool) * scan->numberOfOrderBys);
+	so->distances = palloc(sizeof(so->distances[0]) * scan->numberOfOrderBys);
 	so->qual_ok = true;			/* in case there are zero keys */
 	if (scan->numberOfOrderBys > 0)
 	{

src/backend/access/index/indexam.c

@@ -847,14 +847,14 @@ index_getprocinfo(Relation irel,
  */
 void
 index_store_float8_orderby_distances(IndexScanDesc scan, Oid *orderByTypes,
-									 double *distanceValues,
-									 bool *distanceNulls, bool recheckOrderBy)
+									 IndexOrderByDistance *distances,
+									 bool recheckOrderBy)
 {
 	int			i;
 
 	scan->xs_recheckorderby = recheckOrderBy;
 
-	if (!distanceValues)
+	if (!distances)
 	{
 		Assert(!scan->xs_recheckorderby);
 
@@ -869,20 +869,20 @@ index_store_float8_orderby_distances(IndexScanDesc scan, Oid *orderByTypes,
 
 	for (i = 0; i < scan->numberOfOrderBys; i++)
 	{
-		if (distanceNulls && distanceNulls[i])
-		{
+		scan->xs_orderbynulls[i] = distances[i].isnull;
+
+		if (scan->xs_orderbynulls[i])
 			scan->xs_orderbyvals[i] = (Datum) 0;
-			scan->xs_orderbynulls[i] = true;
-		}
+
 		if (orderByTypes[i] == FLOAT8OID)
 		{
 #ifndef USE_FLOAT8_BYVAL
 			/* must free any old value to avoid memory leakage */
 			if (!scan->xs_orderbynulls[i])
 				pfree(DatumGetPointer(scan->xs_orderbyvals[i]));
 #endif
-			scan->xs_orderbyvals[i] = Float8GetDatum(distanceValues[i]);
-			scan->xs_orderbynulls[i] = false;
+			if (!scan->xs_orderbynulls[i])
+				scan->xs_orderbyvals[i] = Float8GetDatum(distances[i].value);
 		}
 		else if (orderByTypes[i] == FLOAT4OID)
 		{
@@ -892,8 +892,8 @@ index_store_float8_orderby_distances(IndexScanDesc scan, Oid *orderByTypes,
 			if (!scan->xs_orderbynulls[i])
 				pfree(DatumGetPointer(scan->xs_orderbyvals[i]));
 #endif
-			scan->xs_orderbyvals[i] = Float4GetDatum((float4) distanceValues[i]);
-			scan->xs_orderbynulls[i] = false;
+			if (!scan->xs_orderbynulls[i])
+				scan->xs_orderbyvals[i] = Float4GetDatum((float4) distances[i].value);
 		}
 		else
 		{