Merge pull request #5792 from matrix-org/erikj/fix_bg_update

Fix current_state_events membership background update.

changelog.d/5792.misc

@@ -0,0 +1 @@
+Reduce database IO usage by optimising queries for current membership.

synapse/storage/roommember.py

@@ -935,7 +935,7 @@ class RoomMemberStore(RoomMemberWorkerStore):
             while processed < batch_size:
                 txn.execute(
                     """
-                        SELECT MIN(room_id) FROM rooms WHERE room_id > ?
+                        SELECT MIN(room_id) FROM current_state_events WHERE room_id > ?
                     """,
                     (last_processed_room,),
                 )