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, 18 insertions, 0 deletions

diff --git a/src/backend/access/gist/gistscan.c b/src/backend/access/gist/gistscan.c
index 991858ff43f..3522d75a496 100644
--- a/src/backend/access/gist/gistscan.c
+++ b/src/backend/access/gist/gistscan.c
@@ -88,6 +88,13 @@ gistbeginscan(PG_FUNCTION_ARGS)
 
 	scan->opaque = so;
 
+	/*
+	 * All fields required for index-only scans are null until gistrescan.
+	 * However, we set up scan->xs_itupdesc whether we'll need it or not,
+	 * since that's cheap.
+	 */
+	scan->xs_itupdesc = RelationGetDescr(r);
+
 	MemoryContextSwitchTo(oldCxt);
 
 	PG_RETURN_POINTER(scan);
@@ -141,6 +148,17 @@ gistrescan(PG_FUNCTION_ARGS)
 		first_time = false;
 	}
 
+	/*
+	 * If we're doing an index-only scan, also create a memory context to hold
+	 * the returned tuples.
+	 */
+	if (scan->xs_want_itup && so->pageDataCxt == NULL)
+		so->pageDataCxt = AllocSetContextCreate(so->giststate->scanCxt,
+												"GiST page data context",
+												ALLOCSET_DEFAULT_MINSIZE,
+												ALLOCSET_DEFAULT_INITSIZE,
+												ALLOCSET_DEFAULT_MAXSIZE);
+
 	/* create new, empty RBTree for search queue */
 	oldCxt = MemoryContextSwitchTo(so->queueCxt);
 	so->queue = pairingheap_allocate(pairingheap_GISTSearchItem_cmp, scan);