Add an index on sliding_sync_connections.last_used_ts (#19912)

Speeds up finding and deleting expired sliding sync connections in
`delete_old_sliding_sync_connections`, which previously required a
sequential scan.

On matrix.org I have a suspicion that this might end up blocking some
SSS connections during the delete, which can take minutes. Specifically,
I think the deletion blocks this delete:


https://github.com/element-hq/synapse/blob/ff19c034d300869e64878a15aed9a97f1cec59e4/synapse/storage/databases/main/sliding_sync.py#L233-L241

We didn't previously have an index because we wanted the postgres HOT
updates, however we also limit the update frequency to once every 5
minutes, so hopefully this is fine.

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

changelog.d/19912.misc

@@ -0,0 +1 @@
+Speed up deletion of old sliding sync connections by adding an index.

synapse/storage/databases/main/sliding_sync.py

@@ -96,6 +96,14 @@ class SlidingSyncStore(SQLBaseStore):
             replaces_index="sliding_sync_membership_snapshots_user_id",
         )
 
+        self.db_pool.updates.register_background_index_update(
+            update_name="sliding_sync_connections_last_used_ts_idx",
+            index_name="sliding_sync_connections_last_used_ts_idx",
+            table="sliding_sync_connections",
+            columns=("last_used_ts",),
+            where_clause="last_used_ts IS NOT NULL",
+        )
+
         if self.hs.config.worker.run_background_tasks:
             self.clock.looping_call(
                 self.delete_old_sliding_sync_connections,

synapse/storage/schema/main/delta/93/03_sss_pos_last_used.sql

@@ -18,10 +18,3 @@
 -- may want to either backfill this or delete all rows with a NULL value (and
 -- then make it NOT NULL).
 ALTER TABLE sliding_sync_connections ADD COLUMN last_used_ts BIGINT;
-
--- Note: We don't add an index on this column to allow HOT updates on PostgreSQL
--- to reduce the cost of the updates to the column. c.f.
--- https://www.postgresql.org/docs/current/storage-hot.html
---
--- We do query this column directly to find expired connections, but we expect
--- that to be an infrequent operation and a sequential scan should be fine.

synapse/storage/schema/main/delta/94/06_sliding_sync_connections_last_used_ts_index.sql

@@ -0,0 +1,22 @@
+--
+-- This file is licensed under the Affero General Public License (AGPL) version 3.
+--
+-- Copyright (C) 2026 Element Creations Ltd
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU Affero General Public License as
+-- published by the Free Software Foundation, either version 3 of the
+-- License, or (at your option) any later version.
+--
+-- See the GNU Affero General Public License for more details:
+-- <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+
+-- Add an index on `sliding_sync_connections(last_used_ts)` so that finding and
+-- deleting expired connections (in `delete_old_sliding_sync_connections`) does
+-- not require a sequential scan of the table.
+--
+-- This is a partial index as we only ever query for rows with a non-NULL
+-- `last_used_ts`.
+INSERT INTO background_updates (ordering, update_name, progress_json) VALUES
+    (9406, 'sliding_sync_connections_last_used_ts_idx', '{}');