Speed up purge room by adding index (#16657)

What it says on the tin

changelog.d/16657.misc

@@ -0,0 +1 @@
+Speed up purge room by adding an index to `event_push_summary`.

synapse/storage/databases/main/event_push_actions.py

@@ -311,6 +311,14 @@ class EventPushActionsWorkerStore(ReceiptsWorkerStore, StreamWorkerStore, SQLBas
             self._background_drop_null_thread_id_indexes,
         )
 
+        # Add a room ID index to speed up room deletion
+        self.db_pool.updates.register_background_index_update(
+            "event_push_summary_index_room_id",
+            index_name="event_push_summary_index_room_id",
+            table="event_push_summary",
+            columns=["room_id"],
+        )
+
     async def _background_drop_null_thread_id_indexes(
         self, progress: JsonDict, batch_size: int
     ) -> int:

synapse/storage/schema/main/delta/83/06_event_push_summary_room.sql

@@ -0,0 +1,17 @@
+/* Copyright 2023 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.
+ */
+
+INSERT INTO background_updates (ordering, update_name, progress_json) VALUES
+    (8306, 'event_push_summary_index_room_id', '{}');