Add changelog

changelog.d/15735.misc

@@ -0,0 +1 @@
+Simplify query to `_count_known_servers()` for the `synapse_federation_known_servers` metric.

synapse/storage/databases/main/roommember.py

@@ -131,10 +131,10 @@ class RoomMemberWorkerStore(EventsWorkerStore, CacheInvalidationWorkerStore):
                 query = """
                     SELECT COUNT(DISTINCT substr(out.user_id, pos+1))
                     FROM (
-                        SELECT rm.user_id as user_id, instr(rm.user_id, ':')
-                            AS pos FROM room_memberships as rm
-                        INNER JOIN current_state_events as c ON rm.event_id = c.event_id
-                        WHERE c.type = 'm.room.member'
+                        SELECT
+                            user_id AS user_id,
+                            instr(user_id, ':') AS pos
+                        FROM room_memberships
                     ) as out
                 """
             else: