Add another index to device_lists_changes_in_room (#14534)

This helps avoid reading unnecessarily large amounts of data from the
table when querying with a set of room IDs.

changelog.d/14534.misc

@@ -0,0 +1 @@
+Improve DB performance by reducing amount of data that gets read in `device_lists_changes_in_room`.

synapse/storage/databases/main/devices.py

@@ -1441,6 +1441,13 @@ def __init__(
             self._remove_duplicate_outbound_pokes,
         )
 
+        self.db_pool.updates.register_background_index_update(
+            "device_lists_changes_in_room_by_room_index",
+            index_name="device_lists_changes_in_room_by_room_idx",
+            table="device_lists_changes_in_room",
+            columns=["room_id", "stream_id"],
+        )
+
     async def _drop_device_list_streams_non_unique_indexes(
         self, progress: JsonDict, batch_size: int
     ) -> int:

synapse/storage/schema/main/delta/73/13add_device_lists_index.sql

@@ -0,0 +1,20 @@
+/* Copyright 2022 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.
+ */
+
+
+-- Adds an index on `device_lists_changes_in_room (room_id, stream_id)`, which
+-- speeds up `/sync` queries.
+INSERT INTO background_updates (ordering, update_name, progress_json) VALUES
+  (7313, 'device_lists_changes_in_room_by_room_index', '{}');