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Add index to sliding_sync_connection_lazy_members on connection_position #19923

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Add index to sliding_sync_connection_lazy_members on connection_position #19923

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1 change: 1 addition & 0 deletions changelog.d/19923.misc
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Original file line number Diff line number Diff line change
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Add an index to `sliding_sync_connection_lazy_members` to speed up deleting old sliding sync connection positions.

@MadLittleMods MadLittleMods Jul 8, 2026

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This speeds up the cascading delete from sliding_sync_connection_positions, which without an index on connection_position requires a sequential scan of the whole table for each deleted position.

We already have an index on connection_key and we expect only two entries at a time.

Seems weird that an index would help.

8 changes: 8 additions & 0 deletions synapse/storage/databases/main/sliding_sync.py
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Expand Up @@ -104,6 +104,14 @@ def __init__(
where_clause="last_used_ts IS NOT NULL",
)

self.db_pool.updates.register_background_index_update(
update_name="sliding_sync_connection_lazy_members_conn_pos_idx",
index_name="sliding_sync_connection_lazy_members_conn_pos_idx",
table="sliding_sync_connection_lazy_members",
columns=("connection_position",),
where_clause="connection_position IS NOT NULL",
)

if self.hs.config.worker.run_background_tasks:
self.clock.looping_call(
self.delete_old_sliding_sync_connections,
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24 changes: 24 additions & 0 deletions synapse/storage/schema/main/delta/94/07_sliding_sync_lazy_members_position_index.sql
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--
-- 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_connection_lazy_members(connection_position)`
-- so that deleting from `sliding_sync_connection_positions` is efficient. This
-- is needed because `connection_position` has an `ON DELETE CASCADE` foreign key
-- constraint, and without this index Postgres has to sequentially scan the whole
-- table for each deleted position.
--
-- This is a partial index as we only ever need to find rows with a non-NULL
-- `connection_position`.
INSERT INTO background_updates (ordering, update_name, progress_json) VALUES
(9407, 'sliding_sync_connection_lazy_members_conn_pos_idx', '{}');
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