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.

1 files changed, 37 insertions, 0 deletions

diff --git a/src/backend/access/gist/gistproc.c b/src/backend/access/gist/gistproc.c
index 9fab6c87c0..9d21e3fb94 100644
--- a/src/backend/access/gist/gistproc.c
+++ b/src/backend/access/gist/gistproc.c
@@ -152,6 +152,16 @@ gist_box_decompress(PG_FUNCTION_ARGS)
 }
 
 /*
+ * GiST Fetch method for boxes
+ * do not do anything --- we just return the stored box as is.
+ */
+Datum
+gist_box_fetch(PG_FUNCTION_ARGS)
+{
+	PG_RETURN_POINTER(PG_GETARG_POINTER(0));
+}
+
+/*
  * The GiST Penalty method for boxes (also used for points)
  *
  * As in the R-tree paper, we use change in area as our penalty metric
@@ -1186,6 +1196,33 @@ gist_point_compress(PG_FUNCTION_ARGS)
 	PG_RETURN_POINTER(entry);
 }
 
+/*
+ * GiST Fetch method for point
+ *
+ * Get point coordinates from its bounding box coordinates and form new
+ * gistentry.
+ */
+Datum
+gist_point_fetch(PG_FUNCTION_ARGS)
+{
+	GISTENTRY  *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
+	BOX		   *in = DatumGetBoxP(entry->key);
+	Point	   *r;
+	GISTENTRY  *retval;
+
+	retval = palloc(sizeof(GISTENTRY));
+
+	r = (Point *) palloc(sizeof(Point));
+	r->x = in->high.x;
+	r->y = in->high.y;
+	gistentryinit(*retval, PointerGetDatum(r),
+				  entry->rel, entry->page,
+				  entry->offset, FALSE);
+
+	PG_RETURN_POINTER(retval);
+}
+
+
 #define point_point_distance(p1,p2) \
 	DatumGetFloat8(DirectFunctionCall2(point_distance, \
 									   PointPGetDatum(p1), PointPGetDatum(p2)))