Drop (almost) unused index on event_json (#8845)

changelog.d/8845.misc

@@ -0,0 +1 @@
+Drop redundant database index on `event_json`.

synapse/storage/databases/main/purge_events.py

@@ -314,6 +314,7 @@ class PurgeEventsStore(StateGroupWorkerStore, SQLBaseStore):
         for table in (
             "event_auth",
             "event_edges",
+            "event_json",
             "event_push_actions_staging",
             "event_reference_hashes",
             "event_relations",
@@ -340,7 +341,6 @@ class PurgeEventsStore(StateGroupWorkerStore, SQLBaseStore):
             "destination_rooms",
             "event_backward_extremities",
             "event_forward_extremities",
-            "event_json",
             "event_push_actions",
             "event_search",
             "events",

synapse/storage/databases/main/schema/delta/58/24drop_event_json_index.sql

@@ -0,0 +1,19 @@
+/* Copyright 2020 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.
+ */
+
+-- this index is essentially redundant. The only time it was ever used was when purging
+-- rooms - and Synapse 1.24 will change that.
+
+DROP INDEX IF EXISTS event_json_room_id;