Set thread_id column to non-null for event_push_{actions,actions_staging,summary} (#15597)

Updates the database schema to require a thread_id (by adding a
constraint that the column is non-null) for event_push_actions,
event_push_actions_staging, and event_push_actions_summary.

For PostgreSQL we add the constraint as NOT VALID, then
VALIDATE the constraint a background job to avoid locking
the table during an upgrade.

Each table is updated as a separate schema delta to avoid
deadlocks between them.

For SQLite we simply rebuild the table & copy the data.

changelog.d/15597.misc

@@ -0,0 +1 @@
+Make the `thread_id` column on `event_push_actions`, `event_push_actions_staging`, and `event_push_summary` non-null.

synapse/storage/background_updates.py

@@ -561,6 +561,50 @@ class BackgroundUpdater:
             updater, oneshot=True
         )
 
+    def register_background_validate_constraint(
+        self, update_name: str, constraint_name: str, table: str
+    ) -> None:
+        """Helper for store classes to do a background validate constraint.
+
+        This only applies on PostgreSQL.
+
+        To use:
+
+        1. use a schema delta file to add a background update. Example:
+            INSERT INTO background_updates (update_name, progress_json) VALUES
+                ('validate_my_constraint', '{}');
+
+        2. In the Store constructor, call this method
+
+        Args:
+            update_name: update_name to register for
+            constraint_name: name of constraint to validate
+            table: table the constraint is applied to
+        """
+
+        def runner(conn: Connection) -> None:
+            c = conn.cursor()
+
+            sql = f"""
+            ALTER TABLE {table} VALIDATE CONSTRAINT {constraint_name};
+            """
+            logger.debug("[SQL] %s", sql)
+            c.execute(sql)
+
+        async def updater(progress: JsonDict, batch_size: int) -> int:
+            assert isinstance(
+                self.db_pool.engine, engines.PostgresEngine
+            ), "validate constraint background update registered for non-Postres database"
+
+            logger.info("Validating constraint %s to %s", constraint_name, table)
+            await self.db_pool.runWithConnection(runner)
+            await self._end_background_update(update_name)
+            return 1
+
+        self._background_update_handlers[update_name] = _BackgroundUpdateHandler(
+            updater, oneshot=True
+        )
+
     async def create_index_in_background(
         self,
         index_name: str,

synapse/storage/databases/main/event_push_actions.py

@@ -289,179 +289,52 @@ class EventPushActionsWorkerStore(ReceiptsWorkerStore, StreamWorkerStore, SQLBas
             unique=True,
         )
 
-        self.db_pool.updates.register_background_update_handler(
+        self.db_pool.updates.register_background_validate_constraint(
-            "event_push_backfill_thread_id",
+            "event_push_actions_staging_thread_id",
-            self._background_backfill_thread_id,
+            constraint_name="event_push_actions_staging_thread_id",
+            table="event_push_actions_staging",
         )
-
+        self.db_pool.updates.register_background_validate_constraint(
-        # Indexes which will be used to quickly make the thread_id column non-null.
+            "event_push_actions_thread_id",
-        self.db_pool.updates.register_background_index_update(
+            constraint_name="event_push_actions_thread_id",
-            "event_push_actions_thread_id_null",
-            index_name="event_push_actions_thread_id_null",
             table="event_push_actions",
-            columns=["thread_id"],
-            where_clause="thread_id IS NULL",
         )
-        self.db_pool.updates.register_background_index_update(
+        self.db_pool.updates.register_background_validate_constraint(
-            "event_push_summary_thread_id_null",
+            "event_push_summary_thread_id",
-            index_name="event_push_summary_thread_id_null",
+            constraint_name="event_push_summary_thread_id",
             table="event_push_summary",
-            columns=["thread_id"],
-            where_clause="thread_id IS NULL",
         )
 
-        # Check ASAP (and then later, every 1s) to see if we have finished
+        self.db_pool.updates.register_background_update_handler(
-        # background updates the event_push_actions and event_push_summary tables.
+            "event_push_drop_null_thread_id_indexes",
-        self._clock.call_later(0.0, self._check_event_push_backfill_thread_id)
+            self._background_drop_null_thread_id_indexes,
-        self._event_push_backfill_thread_id_done = False
-
-    @wrap_as_background_process("check_event_push_backfill_thread_id")
-    async def _check_event_push_backfill_thread_id(self) -> None:
-        """
-        Has thread_id finished backfilling?
-
-        If not, we need to just-in-time update it so the queries work.
-        """
-        done = await self.db_pool.updates.has_completed_background_update(
-            "event_push_backfill_thread_id"
         )
 
-        if done:
+    async def _background_drop_null_thread_id_indexes(
-            self._event_push_backfill_thread_id_done = True
-        else:
-            # Reschedule to run.
-            self._clock.call_later(15.0, self._check_event_push_backfill_thread_id)
-
-    async def _background_backfill_thread_id(
         self, progress: JsonDict, batch_size: int
     ) -> int:
         """
-        Fill in the thread_id field for event_push_actions and event_push_summary.
+        Drop the indexes used to find null thread_ids for event_push_actions and
-
+        event_push_summary.
-        This is preparatory so that it can be made non-nullable in the future.
-
-        Because all current (null) data is done in an unthreaded manner this
-        simply assumes it is on the "main" timeline. Since event_push_actions
-        are periodically cleared it is not possible to correctly re-calculate
-        the thread_id.
         """
-        event_push_actions_done = progress.get("event_push_actions_done", False)
 
-        def add_thread_id_txn(
+        def drop_null_thread_id_indexes_txn(txn: LoggingTransaction) -> None:
-            txn: LoggingTransaction, start_stream_ordering: int
+            sql = "DROP INDEX IF EXISTS event_push_actions_thread_id_null"
-        ) -> int:
+            logger.debug("[SQL] %s", sql)
-            sql = """
+            txn.execute(sql)
-            SELECT stream_ordering
-            FROM event_push_actions
-            WHERE
-                thread_id IS NULL
-                AND stream_ordering > ?
-            ORDER BY stream_ordering
-            LIMIT ?
-            """
-            txn.execute(sql, (start_stream_ordering, batch_size))
 
-            # No more rows to process.
+            sql = "DROP INDEX IF EXISTS event_push_summary_thread_id_null"
-            rows = txn.fetchall()
+            logger.debug("[SQL] %s", sql)
-            if not rows:
+            txn.execute(sql)
-                progress["event_push_actions_done"] = True
-                self.db_pool.updates._background_update_progress_txn(
-                    txn, "event_push_backfill_thread_id", progress
-                )
-                return 0
 
-            # Update the thread ID for any of those rows.
+        await self.db_pool.runInteraction(
-            max_stream_ordering = rows[-1][0]
+            "drop_null_thread_id_indexes_txn",
-
+            drop_null_thread_id_indexes_txn,
-            sql = """
+        )
-            UPDATE event_push_actions
+        await self.db_pool.updates._end_background_update(
-            SET thread_id = 'main'
+            "event_push_drop_null_thread_id_indexes"
-            WHERE ? < stream_ordering AND stream_ordering <= ? AND thread_id IS NULL
+        )
-            """
+        return 0
-            txn.execute(
-                sql,
-                (
-                    start_stream_ordering,
-                    max_stream_ordering,
-                ),
-            )
-
-            # Update progress.
-            processed_rows = txn.rowcount
-            progress["max_event_push_actions_stream_ordering"] = max_stream_ordering
-            self.db_pool.updates._background_update_progress_txn(
-                txn, "event_push_backfill_thread_id", progress
-            )
-
-            return processed_rows
-
-        def add_thread_id_summary_txn(txn: LoggingTransaction) -> int:
-            min_user_id = progress.get("max_summary_user_id", "")
-            min_room_id = progress.get("max_summary_room_id", "")
-
-            # Slightly overcomplicated query for getting the Nth user ID / room
-            # ID tuple, or the last if there are less than N remaining.
-            sql = """
-            SELECT user_id, room_id FROM (
-                SELECT user_id, room_id FROM event_push_summary
-                WHERE (user_id, room_id) > (?, ?)
-                    AND thread_id IS NULL
-                ORDER BY user_id, room_id
-                LIMIT ?
-            ) AS e
-            ORDER BY user_id DESC, room_id DESC
-            LIMIT 1
-            """
-
-            txn.execute(sql, (min_user_id, min_room_id, batch_size))
-            row = txn.fetchone()
-            if not row:
-                return 0
-
-            max_user_id, max_room_id = row
-
-            sql = """
-            UPDATE event_push_summary
-            SET thread_id = 'main'
-            WHERE
-                (?, ?) < (user_id, room_id) AND (user_id, room_id) <= (?, ?)
-                AND thread_id IS NULL
-            """
-            txn.execute(sql, (min_user_id, min_room_id, max_user_id, max_room_id))
-            processed_rows = txn.rowcount
-
-            progress["max_summary_user_id"] = max_user_id
-            progress["max_summary_room_id"] = max_room_id
-            self.db_pool.updates._background_update_progress_txn(
-                txn, "event_push_backfill_thread_id", progress
-            )
-
-            return processed_rows
-
-        # First update the event_push_actions table, then the event_push_summary table.
-        #
-        # Note that the event_push_actions_staging table is ignored since it is
-        # assumed that items in that table will only exist for a short period of
-        # time.
-        if not event_push_actions_done:
-            result = await self.db_pool.runInteraction(
-                "event_push_backfill_thread_id",
-                add_thread_id_txn,
-                progress.get("max_event_push_actions_stream_ordering", 0),
-            )
-        else:
-            result = await self.db_pool.runInteraction(
-                "event_push_backfill_thread_id",
-                add_thread_id_summary_txn,
-            )
-
-            # Only done after the event_push_summary table is done.
-            if not result:
-                await self.db_pool.updates._end_background_update(
-                    "event_push_backfill_thread_id"
-                )
-
-        return result
 
     async def get_unread_counts_by_room_for_user(self, user_id: str) -> Dict[str, int]:
         """Get the notification count by room for a user. Only considers notifications,
@@ -711,25 +584,6 @@ class EventPushActionsWorkerStore(ReceiptsWorkerStore, StreamWorkerStore, SQLBas
             (ReceiptTypes.READ, ReceiptTypes.READ_PRIVATE),
         )
 
-        # First ensure that the existing rows have an updated thread_id field.
-        if not self._event_push_backfill_thread_id_done:
-            txn.execute(
-                """
-                UPDATE event_push_summary
-                SET thread_id = ?
-                WHERE room_id = ? AND user_id = ? AND thread_id is NULL
-                """,
-                (MAIN_TIMELINE, room_id, user_id),
-            )
-            txn.execute(
-                """
-                UPDATE event_push_actions
-                SET thread_id = ?
-                WHERE room_id = ? AND user_id = ? AND thread_id is NULL
-                """,
-                (MAIN_TIMELINE, room_id, user_id),
-            )
-
         # First we pull the counts from the summary table.
         #
         # We check that `last_receipt_stream_ordering` matches the stream ordering of the
@@ -1545,25 +1399,6 @@ class EventPushActionsWorkerStore(ReceiptsWorkerStore, StreamWorkerStore, SQLBas
                 (room_id, user_id, stream_ordering, *thread_args),
             )
 
-            # First ensure that the existing rows have an updated thread_id field.
-            if not self._event_push_backfill_thread_id_done:
-                txn.execute(
-                    """
-                    UPDATE event_push_summary
-                    SET thread_id = ?
-                    WHERE room_id = ? AND user_id = ? AND thread_id is NULL
-                    """,
-                    (MAIN_TIMELINE, room_id, user_id),
-                )
-                txn.execute(
-                    """
-                    UPDATE event_push_actions
-                    SET thread_id = ?
-                    WHERE room_id = ? AND user_id = ? AND thread_id is NULL
-                    """,
-                    (MAIN_TIMELINE, room_id, user_id),
-                )
-
             # Fetch the notification counts between the stream ordering of the
             # latest receipt and what was previously summarised.
             unread_counts = self._get_notif_unread_count_for_user_room(
@@ -1698,19 +1533,6 @@ class EventPushActionsWorkerStore(ReceiptsWorkerStore, StreamWorkerStore, SQLBas
             rotate_to_stream_ordering: The new maximum event stream ordering to summarise.
         """
 
-        # Ensure that any new actions have an updated thread_id.
-        if not self._event_push_backfill_thread_id_done:
-            txn.execute(
-                """
-                UPDATE event_push_actions
-                SET thread_id = ?
-                WHERE ? < stream_ordering AND stream_ordering <= ? AND thread_id IS NULL
-                """,
-                (MAIN_TIMELINE, old_rotate_stream_ordering, rotate_to_stream_ordering),
-            )
-
-        # XXX Do we need to update summaries here too?
-
         # Calculate the new counts that should be upserted into event_push_summary
         sql = """
             SELECT user_id, room_id, thread_id,
@@ -1773,20 +1595,6 @@ class EventPushActionsWorkerStore(ReceiptsWorkerStore, StreamWorkerStore, SQLBas
 
         logger.info("Rotating notifications, handling %d rows", len(summaries))
 
-        # Ensure that any updated threads have the proper thread_id.
-        if not self._event_push_backfill_thread_id_done:
-            txn.execute_batch(
-                """
-                UPDATE event_push_summary
-                SET thread_id = ?
-                WHERE room_id = ? AND user_id = ? AND thread_id is NULL
-                """,
-                [
-                    (MAIN_TIMELINE, room_id, user_id)
-                    for user_id, room_id, _ in summaries
-                ],
-            )
-
         self.db_pool.simple_upsert_many_txn(
             txn,
             table="event_push_summary",

synapse/storage/schema/__init__.py

@@ -110,6 +110,9 @@ SCHEMA_COMPAT_VERSION = (
     # Queries against `event_stream_ordering` columns in membership tables must
     # be disambiguated.
     #
+    # The threads_id column must written to with non-null values for the
+    # event_push_actions, event_push_actions_staging, and event_push_summary tables.
+    #
     # insertions to the column `full_user_id` of tables profiles and user_filters can no
     # longer be null
     76

synapse/storage/schema/main/delta/77/05thread_notifications_backfill.sql

@@ -0,0 +1,28 @@
+/* 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.
+ */
+
+-- Force the background updates from 06thread_notifications.sql to run in the
+-- foreground as code will now require those to be "done".
+
+DELETE FROM background_updates WHERE update_name = 'event_push_backfill_thread_id';
+
+-- Overwrite any null thread_id values.
+UPDATE event_push_actions_staging SET thread_id = 'main' WHERE thread_id IS NULL;
+UPDATE event_push_actions SET thread_id = 'main' WHERE thread_id IS NULL;
+UPDATE event_push_summary SET thread_id = 'main' WHERE thread_id IS NULL;
+
+-- Drop the background updates to calculate the indexes used to find null thread_ids.
+DELETE FROM background_updates WHERE update_name = 'event_push_actions_thread_id_null';
+DELETE FROM background_updates WHERE update_name = 'event_push_summary_thread_id_null';

synapse/storage/schema/main/delta/77/06thread_notifications_not_null.sql.sqlite

@@ -0,0 +1,102 @@
+/* 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.
+ */
+
+ -- The thread_id columns can now be made non-nullable.
+--
+-- SQLite doesn't support modifying columns to an existing table, so it must
+-- be recreated.
+
+-- Create the new tables.
+CREATE TABLE event_push_actions_staging_new (
+    event_id TEXT NOT NULL,
+    user_id TEXT NOT NULL,
+    actions TEXT NOT NULL,
+    notif SMALLINT NOT NULL,
+    highlight SMALLINT NOT NULL,
+    unread SMALLINT,
+    thread_id TEXT,
+    inserted_ts BIGINT,
+    CONSTRAINT event_push_actions_staging_thread_id CHECK (thread_id is NOT NULL)
+);
+
+CREATE TABLE event_push_actions_new (
+    room_id TEXT NOT NULL,
+    event_id TEXT NOT NULL,
+    user_id TEXT NOT NULL,
+    profile_tag VARCHAR(32),
+    actions TEXT NOT NULL,
+    topological_ordering BIGINT,
+    stream_ordering BIGINT,
+    notif SMALLINT,
+    highlight SMALLINT,
+    unread SMALLINT,
+    thread_id TEXT,
+    CONSTRAINT event_id_user_id_profile_tag_uniqueness UNIQUE (room_id, event_id, user_id, profile_tag),
+    CONSTRAINT event_push_actions_thread_id CHECK (thread_id is NOT NULL)
+);
+
+CREATE TABLE event_push_summary_new (
+    user_id TEXT NOT NULL,
+    room_id TEXT NOT NULL,
+    notif_count BIGINT NOT NULL,
+    stream_ordering BIGINT NOT NULL,
+    unread_count BIGINT,
+    last_receipt_stream_ordering BIGINT,
+    thread_id TEXT,
+    CONSTRAINT event_push_summary_thread_id CHECK (thread_id is NOT NULL)
+);
+
+-- Copy the data.
+INSERT INTO event_push_actions_staging_new (event_id, user_id, actions, notif, highlight, unread, thread_id, inserted_ts)
+    SELECT event_id, user_id, actions, notif, highlight, unread, thread_id, inserted_ts
+    FROM event_push_actions_staging;
+
+INSERT INTO event_push_actions_new (room_id, event_id, user_id, profile_tag, actions, topological_ordering, stream_ordering, notif, highlight, unread, thread_id)
+    SELECT room_id, event_id, user_id, profile_tag, actions, topological_ordering, stream_ordering, notif, highlight, unread, thread_id
+    FROM event_push_actions;
+
+INSERT INTO event_push_summary_new (user_id, room_id, notif_count, stream_ordering, unread_count, last_receipt_stream_ordering, thread_id)
+    SELECT user_id, room_id, notif_count, stream_ordering, unread_count, last_receipt_stream_ordering, thread_id
+    FROM event_push_summary;
+
+-- Drop the old tables.
+DROP TABLE event_push_actions_staging;
+DROP TABLE event_push_actions;
+DROP TABLE event_push_summary;
+
+-- Rename the tables.
+ALTER TABLE event_push_actions_staging_new RENAME TO event_push_actions_staging;
+ALTER TABLE event_push_actions_new RENAME TO event_push_actions;
+ALTER TABLE event_push_summary_new RENAME TO event_push_summary;
+
+-- Recreate the indexes.
+CREATE INDEX event_push_actions_staging_id ON event_push_actions_staging(event_id);
+
+CREATE INDEX event_push_actions_highlights_index ON event_push_actions (user_id, room_id, topological_ordering, stream_ordering);
+CREATE INDEX event_push_actions_rm_tokens on event_push_actions( user_id, room_id, topological_ordering, stream_ordering );
+CREATE INDEX event_push_actions_room_id_user_id on event_push_actions(room_id, user_id);
+CREATE INDEX event_push_actions_stream_ordering on event_push_actions( stream_ordering, user_id );
+CREATE INDEX event_push_actions_u_highlight ON event_push_actions (user_id, stream_ordering);
+
+CREATE UNIQUE INDEX event_push_summary_unique_index2 ON event_push_summary (user_id, room_id, thread_id) ;
+
+-- Recreate some indexes in the background, by re-running the background updates
+-- from 72/02event_push_actions_index.sql and 72/06thread_notifications.sql.
+INSERT INTO background_updates (ordering, update_name, progress_json) VALUES
+  (7706, 'event_push_summary_unique_index2', '{}')
+  ON CONFLICT (update_name) DO UPDATE SET progress_json = '{}';
+INSERT INTO background_updates (ordering, update_name, progress_json) VALUES
+  (7706, 'event_push_actions_stream_highlight_index', '{}')
+  ON CONFLICT (update_name) DO UPDATE SET progress_json = '{}';

synapse/storage/schema/main/delta/77/06thread_notifications_not_null_event_push_actions.sql.postgres

@@ -0,0 +1,27 @@
+/* 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.
+ */
+
+-- The thread_id columns can now be made non-nullable, this is done by using a
+-- constraint (and not altering the column) to avoid taking out a full table lock.
+--
+-- We initially add an invalid constraint which guards against new data (this
+-- doesn't lock the table).
+ALTER TABLE event_push_actions
+    ADD CONSTRAINT event_push_actions_thread_id CHECK (thread_id IS NOT NULL) NOT VALID;
+
+-- We then validate the constraint which doesn't need to worry about new data. It
+-- only needs a SHARE UPDATE EXCLUSIVE lock but can still take a while to complete.
+INSERT INTO background_updates (ordering, update_name, progress_json, depends_on) VALUES
+  (7706, 'event_push_actions_thread_id', '{}', 'event_push_actions_staging_thread_id');

synapse/storage/schema/main/delta/77/06thread_notifications_not_null_event_push_actions_staging.sql.postgres

@@ -0,0 +1,27 @@
+/* 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.
+ */
+
+-- The thread_id columns can now be made non-nullable, this is done by using a
+-- constraint (and not altering the column) to avoid taking out a full table lock.
+--
+-- We initially add an invalid constraint which guards against new data (this
+-- doesn't lock the table).
+ALTER TABLE event_push_actions_staging
+    ADD CONSTRAINT event_push_actions_staging_thread_id CHECK (thread_id IS NOT NULL) NOT VALID;
+
+-- We then validate the constraint which doesn't need to worry about new data. It
+-- only needs a SHARE UPDATE EXCLUSIVE lock but can still take a while to complete.
+INSERT INTO background_updates (ordering, update_name, progress_json) VALUES
+  (7706, 'event_push_actions_staging_thread_id', '{}');

synapse/storage/schema/main/delta/77/06thread_notifications_not_null_event_push_summary.sql.postgres

@@ -0,0 +1,29 @@
+/* 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.
+ */
+
+-- The thread_id columns can now be made non-nullable, this is done by using a
+-- constraint (and not altering the column) to avoid taking out a full table lock.
+--
+-- We initially add an invalid constraint which guards against new data (this
+-- doesn't lock the table).
+ALTER TABLE event_push_summary
+    ADD CONSTRAINT event_push_summary_thread_id CHECK (thread_id IS NOT NULL) NOT VALID;
+
+-- We then validate the constraint which doesn't need to worry about new data. It
+-- only needs a SHARE UPDATE EXCLUSIVE lock but can still take a while to complete.
+INSERT INTO background_updates (ordering, update_name, progress_json, depends_on) VALUES
+  (7706, 'event_push_summary_thread_id', '{}', 'event_push_actions_thread_id'),
+  -- Also clean-up the old indexes.
+  (7706, 'event_push_drop_null_thread_id_indexes', '{}', 'event_push_summary_thread_id');