Don't recreate column

synapse/storage/schema/delta/56/current_state_events_membership.sql

+/* Copyright 2019 The Matrix.org Foundation C.I.C.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+-- We add membership to current state so that we don't need to join against
+-- room_memberships, which can be surprisingly costly (we do such queries
+-- very frequently).
+-- This will be null for non-membership events and the content.membership key
+-- for membership events. (Will also be null for membership events until the
+-- background update job has finished).
+ALTER TABLE current_state_events ADD membership TEXT;

synapse/storage/schema/delta/56/current_state_events_membership_mk2.sql

@@ -19,7 +19,6 @@
 -- This will be null for non-membership events and the content.membership key
 -- for membership events. (Will also be null for membership events until the
 -- background update job has finished).
-ALTER TABLE current_state_events ADD membership TEXT;
 
 INSERT INTO background_updates (update_name, progress_json) VALUES
   ('current_state_events_membership', '{}');