Make get_joined_users faster

synapse/storage/roommember.py

@@ -417,25 +417,51 @@ def _get_joined_users_from_context(self, room_id, state_group, current_state_ids
             if key[0] == EventTypes.Member
         ]
 
-        rows = yield self._simple_select_many_batch(
-            table="room_memberships",
-            column="event_id",
-            iterable=member_event_ids,
-            retcols=['user_id', 'display_name', 'avatar_url'],
-            keyvalues={
-                "membership": Membership.JOIN,
-            },
-            batch_size=500,
-            desc="_get_joined_users_from_context",
+        # We check if we have any of the member event ids in the event cache
+        # before we ask the DB
+
+        event_map = self._get_events_from_cache(
+            member_event_ids,
+            allow_rejected=False,
         )
 
-        users_in_room = {
-            to_ascii(row["user_id"]): ProfileInfo(
-                avatar_url=to_ascii(row["avatar_url"]),
-                display_name=to_ascii(row["display_name"]),
+        missing_member_event_ids = []
+        users_in_room = {}
+        for event_id in member_event_ids:
+            ev_entry = event_map.get(event_id)
+            if ev_entry:
+                if ev_entry.event.membership == Membership.JOIN:
+                    users_in_room[to_ascii(ev_entry.event.state_key)] = ProfileInfo(
+                        display_name=to_ascii(
+                            ev_entry.event.content.get("displayname", None)
+                        ),
+                        avatar_url=to_ascii(
+                            ev_entry.event.content.get("avatar_url", None)
+                        ),
+                    )
+            else:
+                missing_member_event_ids.append(event_id)
+
+        if missing_member_event_ids:
+            rows = yield self._simple_select_many_batch(
+                table="room_memberships",
+                column="event_id",
+                iterable=missing_member_event_ids,
+                retcols=('user_id', 'display_name', 'avatar_url',),
+                keyvalues={
+                    "membership": Membership.JOIN,
+                },
+                batch_size=500,
+                desc="_get_joined_users_from_context",
             )
-            for row in rows
-        }
+
+            users_in_room.update({
+                to_ascii(row["user_id"]): ProfileInfo(
+                    avatar_url=to_ascii(row["avatar_url"]),
+                    display_name=to_ascii(row["display_name"]),
+                )
+                for row in rows
+            })
 
         if event is not None and event.type == EventTypes.Member:
             if event.membership == Membership.JOIN: