Microvacuum for GIST

Mark index tuple as dead if it's pointed by kill_prior_tuple during
ordinary (search) scan and remove it during insert process if there is no
enough space for new tuple to insert. This improves select performance
because index will not return tuple marked as dead and improves insert
performance because it reduces number of page split.

Anastasia Lubennikova <a.lubennikova@postgrespro.ru> with
 minor editorialization by me

src/include/access/gist_private.h

@@ -22,6 +22,7 @@
 #include "storage/bufmgr.h"
 #include "storage/buffile.h"
 #include "utils/hsearch.h"
+#include "access/genam.h"
 
 /*
  * Maximum number of "halves" a page can be split into in one operation.
@@ -121,9 +122,11 @@ typedef struct GISTSearchHeapItem
 {
 	ItemPointerData heapPtr;
 	bool		recheck;		/* T if quals must be rechecked */
-	bool		recheckDistances;		/* T if distances must be rechecked */
+	bool		recheckDistances;	/* T if distances must be rechecked */
 	IndexTuple	ftup;			/* data fetched back from the index, used in
 								 * index-only scans */
+	OffsetNumber	offnum;		/* track offset in page to mark tuple as
+								 * LP_DEAD */
 } GISTSearchHeapItem;
 
 /* Unvisited item, either index page or heap tuple */
@@ -161,6 +164,12 @@ typedef struct GISTScanOpaqueData
 	/* pre-allocated workspace arrays */
 	double	   *distances;		/* output area for gistindex_keytest */
 
+	/* info about killed items if any (killedItems is NULL if never used) */
+	OffsetNumber *killedItems;		/* offset numbers of killed items */
+	int			numKilled;		/* number of currently stored items */
+	BlockNumber curBlkno;		/* current number of block */
+	GistNSN		curPageLSN;	/* pos in the WAL stream when page was read */
+
 	/* In a non-ordered search, returnable heap items are stored here: */
 	GISTSearchHeapItem pageData[BLCKSZ / sizeof(IndexTupleData)];
 	OffsetNumber nPageData;		/* number of valid items in array */