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, 8 insertions, 4 deletions

diff --git a/src/backend/access/index/indexam.c b/src/backend/access/index/indexam.c
index 00c1d693766..2b27e732f13 100644
--- a/src/backend/access/index/indexam.c
+++ b/src/backend/access/index/indexam.c
@@ -722,11 +722,14 @@ index_vacuum_cleanup(IndexVacuumInfo *info,
 }
 
 /* ----------------
- *		index_can_return - does index support index-only scans?
+ *		index_can_return
+ *
+ *		Does the index access method support index-only scans for the given
+ *		column?
  * ----------------
  */
 bool
-index_can_return(Relation indexRelation)
+index_can_return(Relation indexRelation, int attno)
 {
 	FmgrInfo   *procedure;
 
@@ -738,8 +741,9 @@ index_can_return(Relation indexRelation)
 
 	GET_REL_PROCEDURE(amcanreturn);
 
-	return DatumGetBool(FunctionCall1(procedure,
-									  PointerGetDatum(indexRelation)));
+	return DatumGetBool(FunctionCall2(procedure,
+									  PointerGetDatum(indexRelation),
+									  Int32GetDatum(attno)));
 }
 
 /* ----------------