Make simple query rather than long one and then throw away half the results

synapse/handlers/message.py

@@ -267,8 +267,7 @@ class MessageHandler(BaseHandler):
             user, pagination_config.get_source_config("presence"), None
         )
 
-        public_rooms = yield self.store.get_rooms(is_public=True)
-        public_room_ids = [r["room_id"] for r in public_rooms]
+        public_room_ids = yield self.store.get_public_room_ids()
 
         limit = pagin_config.limit
         if limit is None:

synapse/storage/room.py

@@ -75,6 +75,16 @@ class RoomStore(SQLBaseStore):
             allow_none=True,
         )
 
+    def get_public_room_ids(self):
+        return self._simple_select_onecol(
+            table="rooms",
+            keyvalues={
+                "is_public": True,
+            },
+            retcol="room_id",
+            desc="get_public_room_ids",
+        )
+
     @defer.inlineCallbacks
     def get_rooms(self, is_public):
         """Retrieve a list of all public rooms.