Short circuit if all new events have same state group

synapse/storage/events.py

@@ -428,6 +428,7 @@ class EventsStore(SQLBaseStore):
         # Now we need to work out the different state sets for
         # each state extremities
         state_sets = []
+        state_groups = set()
         missing_event_ids = []
         was_updated = False
         for event_id in new_latest_event_ids:
@@ -437,9 +438,17 @@ class EventsStore(SQLBaseStore):
                 if event_id == ev.event_id:
                     if ctx.current_state_ids is None:
                         raise Exception("Unknown current state")
+
+                    # If we've akready seen the state group don't bother adding
+                    # it to the state sets again
+                    if ctx.state_group not in state_groups:
                         state_sets.append(ctx.current_state_ids)
                         if ctx.delta_ids or hasattr(ev, "state_key"):
                             was_updated = True
+                        if ctx.state_group:
+                            # Add this as a seen state group (if it has a state
+                            # group)
+                            state_groups.add(ctx.state_group)
                     break
             else:
                 # If we couldn't find it, then we'll need to pull
@@ -453,14 +462,20 @@ class EventsStore(SQLBaseStore):
                 missing_event_ids,
             )
 
-            groups = set(event_to_groups.itervalues())
-            group_to_state = yield self._get_state_for_groups(groups)
+            groups = set(event_to_groups.itervalues()) - state_groups
 
+            if groups:
+                group_to_state = yield self._get_state_for_groups(groups)
                 state_sets.extend(group_to_state.itervalues())
 
         if not new_latest_event_ids:
             current_state = {}
         elif was_updated:
+            if len(state_sets) == 1:
+                # If there is only one state set, then we know what the current
+                # state is.
+                current_state = state_sets[0]
+            else:
                 # We work out the current state by passing the state sets to the
                 # state resolution algorithm. It may ask for some events, including
                 # the events we have yet to persist, so we need a slightly more