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

This commit improves subject in two ways:

 * It removes ugliness of 02f90879e7, 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

src/include/access/gist_private.h

@@ -14,6 +14,7 @@
 #ifndef GIST_PRIVATE_H
 #define GIST_PRIVATE_H
 
+#include "access/genam.h"
 #include "access/gist.h"
 #include "access/itup.h"
 #include "fmgr.h"
@@ -145,27 +146,13 @@ typedef struct GISTSearchTreeItem
 	GISTSearchItem *head;		/* first chain member */
 	GISTSearchItem *lastHeap;	/* last heap-tuple member, if any */
 
-	/*
-	 * This data structure is followed by arrays of distance values and
-	 * distance null flags.  Size of both arrays is
-	 * IndexScanDesc->numberOfOrderBys. See macros below for accessing those
-	 * arrays.
-	 */
+	/* numberOfOrderBys entries */
+	IndexOrderByDistance distances[FLEXIBLE_ARRAY_MEMBER];
 } GISTSearchTreeItem;
 
-#define SizeOfGISTSearchTreeItem(n_distances) (DOUBLEALIGN(sizeof(GISTSearchTreeItem)) + \
-	(sizeof(double) + sizeof(bool)) * (n_distances))
-
-/*
- * We actually don't need n_distances compute pointer to distance values.
- * Nevertheless take n_distances as argument to have same arguments list for
- * GISTSearchItemDistanceValues() and GISTSearchItemDistanceNulls().
- */
-#define GISTSearchTreeItemDistanceValues(item, n_distances) \
-	((double *) ((Pointer) (item) + DOUBLEALIGN(sizeof(GISTSearchTreeItem))))
-
-#define GISTSearchTreeItemDistanceNulls(item, n_distances) \
-	((bool *) ((Pointer) (item) + DOUBLEALIGN(sizeof(GISTSearchTreeItem)) + sizeof(double) * (n_distances)))
+#define SizeOfGISTSearchTreeItem(n_distances) \
+	(offsetof(GISTSearchTreeItem, distances) + \
+	 sizeof(IndexOrderByDistance) * (n_distances))
 
 /*
  * GISTScanOpaqueData: private state for a scan of a GiST index
@@ -182,8 +169,7 @@ typedef struct GISTScanOpaqueData
 
 	/* pre-allocated workspace arrays */
 	GISTSearchTreeItem *tmpTreeItem;	/* workspace to pass to rb_insert */
-	double	   *distanceValues; /* output area for gistindex_keytest */
-	bool	   *distanceNulls;
+	IndexOrderByDistance *distances;	/* output area for gistindex_keytest */
 
 	/* In a non-ordered search, returnable heap items are stored here: */
 	GISTSearchHeapItem pageData[BLCKSZ / sizeof(IndexTupleData)];