diff --git a/synapse/storage/schema/main/delta/92/10_thread_updates_indexes.sql b/synapse/storage/schema/main/delta/92/10_thread_updates_indexes.sql
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+--
+-- This file is licensed under the Affero General Public License (AGPL) version 3.
+--
+-- Copyright (C) 2025 New Vector, 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 indexes to improve performance of the thread_updates endpoint and
+-- sliding sync threads extension (MSC4360).
+
+-- Index for efficiently finding all events that relate to a specific event
+-- (e.g., all replies to a thread root). This is used by the correlated subquery
+-- in get_thread_updates_for_user that counts thread updates.
+-- Also useful for other relation queries (edits, reactions, etc.).
+CREATE INDEX IF NOT EXISTS event_relations_relates_to_id_type
+    ON event_relations(relates_to_id, relation_type);
+
+-- Index for the /thread_updates endpoint's cross-room query.
+-- Allows efficient descending ordering and range filtering of threads
+-- by stream_ordering across all rooms.
+CREATE INDEX IF NOT EXISTS threads_stream_ordering_desc
+    ON threads(stream_ordering DESC);
+
+-- Index for the EXISTS clause that filters threads to only joined rooms.
+-- Allows efficient lookup of a user's current room memberships.
+CREATE INDEX IF NOT EXISTS local_current_membership_user_room
+    ON local_current_membership(user_id, membership, room_id);