Add support for index-only scans in GiST.

This adds a new GiST opclass method, 'fetch', which is used to reconstruct
the original Datum from the value stored in the index. Also, the 'canreturn'
index AM interface function gains a new 'attno' argument. That makes it
possible to use index-only scans on a multi-column index where some of the
opclasses support index-only scans but some do not.

This patch adds support in the box and point opclasses. Other opclasses
can added later as follow-on patches (btree_gist would be particularly
interesting).

Anastasia Lubennikova, with additional fixes and modifications by me.

3 files changed, 11 insertions, 3 deletions

diff --git a/src/include/access/genam.h b/src/include/access/genam.h
index d1d624721db..d86590ac111 100644
--- a/src/include/access/genam.h
+++ b/src/include/access/genam.h
@@ -156,7 +156,7 @@ extern IndexBulkDeleteResult *index_bulk_delete(IndexVacuumInfo *info,
 				  void *callback_state);
 extern IndexBulkDeleteResult *index_vacuum_cleanup(IndexVacuumInfo *info,
 					 IndexBulkDeleteResult *stats);
-extern bool index_can_return(Relation indexRelation);
+extern bool index_can_return(Relation indexRelation, int attno);
 extern RegProcedure index_getprocid(Relation irel, AttrNumber attnum,
 				uint16 procnum);
 extern FmgrInfo *index_getprocinfo(Relation irel, AttrNumber attnum,
diff --git a/src/include/access/gist.h b/src/include/access/gist.h
index 01f0a70feee..50261b8bdd5 100644
--- a/src/include/access/gist.h
+++ b/src/include/access/gist.h
@@ -33,7 +33,8 @@
 #define GIST_PICKSPLIT_PROC				6
 #define GIST_EQUAL_PROC					7
 #define GIST_DISTANCE_PROC				8
-#define GISTNProcs						8
+#define GIST_FETCH_PROC					9
+#define GISTNProcs					9
 
 /*
  * strategy numbers for GiST opclasses that want to implement the old
diff --git a/src/include/access/gist_private.h b/src/include/access/gist_private.h
index 95506879168..3693893e261 100644
--- a/src/include/access/gist_private.h
+++ b/src/include/access/gist_private.h
@@ -87,6 +87,7 @@ typedef struct GISTSTATE
 	FmgrInfo	picksplitFn[INDEX_MAX_KEYS];
 	FmgrInfo	equalFn[INDEX_MAX_KEYS];
 	FmgrInfo	distanceFn[INDEX_MAX_KEYS];
+	FmgrInfo	fetchFn[INDEX_MAX_KEYS];
 
 	/* Collations to pass to the support functions */
 	Oid			supportCollation[INDEX_MAX_KEYS];
@@ -118,6 +119,8 @@ typedef struct GISTSearchHeapItem
 {
 	ItemPointerData heapPtr;
 	bool		recheck;		/* T if quals must be rechecked */
+	IndexTuple	ftup;			/* data fetched back from the index, used in
+								 * index-only scans */
 } GISTSearchHeapItem;
 
 /* Unvisited item, either index page or heap tuple */
@@ -157,6 +160,8 @@ typedef struct GISTScanOpaqueData
 	GISTSearchHeapItem pageData[BLCKSZ / sizeof(IndexTupleData)];
 	OffsetNumber nPageData;		/* number of valid items in array */
 	OffsetNumber curPageData;	/* next item to return */
+	MemoryContext pageDataCxt;	/* context holding the fetched tuples, for
+								   index-only scans */
 } GISTScanOpaqueData;
 
 typedef GISTScanOpaqueData *GISTScanOpaque;
@@ -409,6 +414,7 @@ typedef struct GiSTOptions
 /* gist.c */
 extern Datum gistbuildempty(PG_FUNCTION_ARGS);
 extern Datum gistinsert(PG_FUNCTION_ARGS);
+extern Datum gistcanreturn(PG_FUNCTION_ARGS);
 extern MemoryContext createTempGistContext(void);
 extern GISTSTATE *initGISTstate(Relation index);
 extern void freeGISTstate(GISTSTATE *giststate);
@@ -504,7 +510,8 @@ extern void gistMakeUnionItVec(GISTSTATE *giststate, IndexTuple *itvec, int len,
 extern bool gistKeyIsEQ(GISTSTATE *giststate, int attno, Datum a, Datum b);
 extern void gistDeCompressAtt(GISTSTATE *giststate, Relation r, IndexTuple tuple, Page p,
 				  OffsetNumber o, GISTENTRY *attdata, bool *isnull);
-
+extern IndexTuple gistFetchTuple(GISTSTATE *giststate, Relation r,
+			   IndexTuple tuple);
 extern void gistMakeUnionKey(GISTSTATE *giststate, int attno,
 				 GISTENTRY *entry1, bool isnull1,
 				 GISTENTRY *entry2, bool isnull2,