Optimise push action processing

This query is responsible for a significant amount of load on our instance and I’d like to optimise it 😃

My first (currently only 😃) suggestion is to store the stream ordering of an event in the receipts_linearized table (probably as event_stream_ordering). I believe (will confirm at a later time) that this means both queries can be combined into a much simpler one along the lines of:

SELECT ep.event_id, ep.room_id, ep.stream_ordering, ep.actions,
   ep.highlight
 FROM event_push_actions AS ep
 LEFT JOIN receipts_linearized AS rl USING (room_id, event_id)
 WHERE
   ep.stream_ordering > rl.stream_ordering
   AND ep.user_id = ?
   AND ep.stream_ordering > ?
   AND ep.stream_ordering <= ?
   AND ep.notif = 1
 ORDER BY ep.stream_ordering ASC LIMIT ?

This would also optimise a bunch of other queries such as here and here.

[Fizzadar]

Progress update: we are using https://github.com/matrix-org/synapse/pull/13918 in our fork and this has yielded significant improvement to the IOPs at all times and in particular during posts in large rooms.

Next up to this is improving the counting of unread actions used when sending pushes: https://github.com/matrix-org/synapse/issues/13846

[anoadragon453]

Adding to to-discuss for the team to consider as another method to optimise event push action processing.