Newsfile

In certain cases going from a stream token to a topological token can be
expensive (as it all rows in the room above the stream ordering), so we
refactor things so we do that calculation less.

changelog.d/17055.misc

@@ -0,0 +1 @@
+Reduce database usage when backpaginating.

synapse/handlers/pagination.py

@@ -468,16 +468,6 @@ class PaginationHandler:
                 room_id, requester, allow_departed_users=True
             )
 
-        if pagin_config.direction == Direction.BACKWARDS:
-            # if we're going backwards, we might need to backfill. This
-            # requires that we have a topo token.
-            if room_token.topological:
-                curr_topo = room_token.topological
-            else:
-                curr_topo = await self.store.get_current_topological_token(
-                    room_id, room_token.stream
-                )
-
         # If they have left the room then clamp the token to be before
         # they left the room, to save the effort of loading from the
         # database.
@@ -496,6 +486,14 @@ class PaginationHandler:
             )
             assert leave_token.topological is not None
 
+            # We need the topological part of the token to compare against.
+            if room_token.topological:
+                curr_topo = room_token.topological
+            else:
+                curr_topo = await self.store.get_current_topological_token(
+                    room_id, room_token.stream
+                )
+
             if leave_token.topological < curr_topo:
                 from_token = from_token.copy_and_replace(
                     StreamKeyType.ROOM, leave_token
@@ -561,6 +559,19 @@ class PaginationHandler:
                     break
                 previous_event_depth = event_depth
 
+            # if we're going backwards, we might need to backfill. This
+            # requires that we have a topo token.
+            if room_token.topological:
+                curr_topo = room_token.topological
+            elif events:
+                # If we've already fetched some events then we can just use
+                # those to get the right depth.
+                curr_topo = events[0].depth
+            else:
+                curr_topo = await self.store.get_current_topological_token(
+                    room_id, room_token.stream
+                )
+
             # Backfill in the foreground if we found a big gap, have too many holes,
             # or we don't have enough events to fill the limit that the client asked
             # for.