Replace simple constant pg_am.amcanreturn with an AM support function.

The need for this was debated when we put in the index-only-scan feature,
but at the time we had no near-term expectation of having AMs that could
support such scans for only some indexes; so we kept it simple.  However,
the SP-GiST AM forces the issue, so let's fix it.

This patch only installs the new API; no behavior actually changes.

3 files changed, 40 insertions, 0 deletions

diff --git a/src/backend/access/index/indexam.c b/src/backend/access/index/indexam.c
index 6d423a7d682..e5fb5183897 100644
--- a/src/backend/access/index/indexam.c
+++ b/src/backend/access/index/indexam.c
@@ -26,6 +26,7 @@
  *		index_getbitmap - get all tuples from a scan
  *		index_bulk_delete	- bulk deletion of index tuples
  *		index_vacuum_cleanup	- post-deletion cleanup of an index
+ *		index_can_return	- does index support index-only scans?
  *		index_getprocid - get a support procedure OID
  *		index_getprocinfo - get a support procedure's lookup info
  *
@@ -712,6 +713,27 @@ index_vacuum_cleanup(IndexVacuumInfo *info,
 }
 
 /* ----------------
+ *		index_can_return - does index support index-only scans?
+ * ----------------
+ */
+bool
+index_can_return(Relation indexRelation)
+{
+	FmgrInfo   *procedure;
+
+	RELATION_CHECKS;
+
+	/* amcanreturn is optional; assume FALSE if not provided by AM */
+	if (!RegProcedureIsValid(indexRelation->rd_am->amcanreturn))
+		return false;
+
+	GET_REL_PROCEDURE(amcanreturn);
+
+	return DatumGetBool(FunctionCall1(procedure,
+									  PointerGetDatum(indexRelation)));
+}
+
+/* ----------------
  *		index_getprocid
  *
  *		Index access methods typically require support routines that are
diff --git a/src/backend/access/nbtree/nbtree.c b/src/backend/access/nbtree/nbtree.c
index f3a1d256a05..13c05525179 100644
--- a/src/backend/access/nbtree/nbtree.c
+++ b/src/backend/access/nbtree/nbtree.c
@@ -1091,3 +1091,14 @@ restart:
 		goto restart;
 	}
 }
+
+/*
+ *	btcanreturn() -- Check whether btree indexes support index-only scans.
+ *
+ * btrees always do, so this is trivial.
+ */
+Datum
+btcanreturn(PG_FUNCTION_ARGS)
+{
+	PG_RETURN_BOOL(true);
+}
diff --git a/src/backend/access/spgist/spgscan.c b/src/backend/access/spgist/spgscan.c
index ac309649682..748265ecba7 100644
--- a/src/backend/access/spgist/spgscan.c
+++ b/src/backend/access/spgist/spgscan.c
@@ -559,3 +559,10 @@ spggettuple(PG_FUNCTION_ARGS)
 
 	PG_RETURN_BOOL(false);
 }
+
+Datum
+spgcanreturn(PG_FUNCTION_ARGS)
+{
+	/* Not implemented yet */
+	PG_RETURN_BOOL(false);
+}