Fix get_last_event_in_room_before_stream_ordering(...) finding the wrong last event

[MadLittleMods]

Found while working on Sliding Sync and split out from #17293

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Fix get_last_event_in_room_before_stream_ordering(...) finding the wrong last event.

PR where this was introduced: matrix-org/synapse#14817

What does this affect?

get_last_event_in_room_before_stream_ordering(...) is used in Sync v2 in a lot of different state calculations.

get_last_event_in_room_before_stream_ordering(...) is also used in /rooms/{roomId}/members

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
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  (run the linters)

[MadLittleMods]

Previously, the query would return rows in stream_ordering ascending order because UNION does not preserve ordering of the operand queries. UNION also behaves as a union + DISTINCT whereas UNION ALL is just the union behavior you're probably expecting.

Before (ascending):

instance_name=worker1, stream_ordering=8 event_id=$MGVB0M_oj6o7FOjpRivsh7UjxQjuLy3ic8Wo5b3cbYU
instance_name=worker1, stream_ordering=9 event_id=$qTr48e2-uDwwKqVH5YJ1r1brPw9jJA4jUnHClGGPyj0

Since we don't need that DISTINCT behavior (because our sub-queries are mutually exclusive anyway and duplicates wouldn't matter anyway), we are now using UNION ALL which preserves the order of the operand queries (works correctly by itself in my testing) but may not preserve the order in all cases, with different indexes or partitioning or a parallel plan. So we add an extra ORDER BY stream_ordering DESC to the end to give us the actual guarantee.

After (descending):

instance_name=worker1, stream_ordering=9 event_id=$xuRTTdLJcUnjMea6JJb-9wDD4eMA9bXaazs0yzNRKxw
instance_name=worker1, stream_ordering=8 event_id=$dBXh-nyRaTq4NTvDAKQZMKnIev3blRmtJV4BqWzRYB4

See https://dba.stackexchange.com/questions/316818/are-results-from-union-all-clauses-always-appended-in-order/316835#316835 for more info on the UNION ALL order guarantee.

[MadLittleMods]

Thanks for the review @erikjohnston 🦒