Fix a performance issue introduced in Synapse v1.83.0 which meant that purging rooms was very slow and database-intensive.

changelog.d/15693.bugfix

@@ -0,0 +1 @@
+Fix a performance issue introduced in Synapse v1.83.0 which meant that purging rooms was very slow and database-intensive.

synapse/storage/databases/state/bg_updates.py

@@ -261,6 +261,16 @@ class StateBackgroundUpdateStore(StateGroupBackgroundUpdateStore):
     STATE_GROUPS_ROOM_INDEX_UPDATE_NAME = "state_groups_room_id_idx"
     STATE_GROUP_EDGES_UNIQUE_INDEX_UPDATE_NAME = "state_group_edges_unique_idx"
 
+    CURRENT_STATE_EVENTS_STREAM_ORDERING_INDEX_UPDATE_NAME = (
+        "current_state_events_stream_ordering_idx"
+    )
+    ROOM_MEMBERSHIPS_STREAM_ORDERING_INDEX_UPDATE_NAME = (
+        "room_memberships_stream_ordering_idx"
+    )
+    LOCAL_CURRENT_MEMBERSHIP_STREAM_ORDERING_INDEX_UPDATE_NAME = (
+        "local_current_membership_stream_ordering_idx"
+    )
+
     def __init__(
         self,
         database: DatabasePool,
@@ -297,6 +307,27 @@ def __init__(
             replaces_index="state_group_edges_idx",
         )
 
+        # These indices are needed to validate the foreign key constraint
+        # when events are deleted.
+        self.db_pool.updates.register_background_index_update(
+            self.CURRENT_STATE_EVENTS_STREAM_ORDERING_INDEX_UPDATE_NAME,
+            index_name="current_state_events_stream_ordering_idx",
+            table="current_state_events",
+            columns=["event_stream_ordering"],
+        )
+        self.db_pool.updates.register_background_index_update(
+            self.ROOM_MEMBERSHIPS_STREAM_ORDERING_INDEX_UPDATE_NAME,
+            index_name="room_memberships_stream_ordering_idx",
+            table="room_memberships",
+            columns=["event_stream_ordering"],
+        )
+        self.db_pool.updates.register_background_index_update(
+            self.LOCAL_CURRENT_MEMBERSHIP_STREAM_ORDERING_INDEX_UPDATE_NAME,
+            index_name="local_current_membership_stream_ordering_idx",
+            table="local_current_membership",
+            columns=["event_stream_ordering"],
+        )
+
     async def _background_deduplicate_state(
         self, progress: dict, batch_size: int
     ) -> int:

synapse/storage/schema/main/delta/77/14bg_indices_event_stream_ordering.sql

@@ -0,0 +1,20 @@
+/* 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
+        (7714, 'current_state_events_stream_ordering_idx', '{}'),
+        (7714, 'local_current_membership_stream_ordering_idx', '{}'),
+        (7714, 'room_memberships_stream_ordering_idx', '{}');