Fix incredubly slow back pagination query

If a client didn't specify a from token when paginating backwards
synapse would attempt to query the (global) maximum topological token.
This a) doesn't make much sense since they're room specific and b) there
are no indices that lets postgres do this efficiently.

2 files changed, 7 insertions, 4 deletions

diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 59eb26beaf..abfa8c65a4 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -82,8 +82,8 @@ class MessageHandler(BaseHandler):
             room_token = pagin_config.from_token.room_key
         else:
             pagin_config.from_token = (
-                yield self.hs.get_event_sources().get_current_token(
-                    direction='b'
+                yield self.hs.get_event_sources().get_current_token_for_room(
+                    room_id=room_id
                 )
             )
             room_token = pagin_config.from_token.room_key
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index a7f533f7be..59e4d1cd15 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -475,8 +475,11 @@ class RoomEventSource(object):
 
         defer.returnValue((events, end_key))
 
-    def get_current_key(self, direction='f'):
-        return self.store.get_room_events_max_id(direction)
+    def get_current_key(self):
+        return self.store.get_room_events_max_id()
+
+    def get_current_key_for_room(self, room_id):
+        return self.store.get_room_events_max_id(room_id)
 
     @defer.inlineCallbacks
     def get_pagination_rows(self, user, config, key):