Improve performance of query _get_subset_users_in_room_with_profiles

[H-Shay]

While profiling local joins I noticed that this query (which was just added) appeared to be quite slow, which was confirmed by the query planner. This PR attempts to remedy that.

[DMRobertson]

Could you post the query plan before and after, just for completeness?

[DMRobertson]

I noticed that this query (which was just added) appeared to be quite slow,

How slow, out of interest---got any numbers?

[H-Shay]

Original query plan:

Nested Loop  (cost=453.40..459.45 rows=1 width=78) (actual time=5.132..5.134 rows=0 loops=1)
   ->  Bitmap Heap Scan on room_memberships m  (cost=452.71..454.73 rows=1 width=156) (actual time=5.131..5.132 rows=0 
loops=1)
         Recheck Cond: ((user_id = ANY ('{@redacted:matrix.org,@redacted2t:matrix.org}'::text[])) AND (room_id = '!redacted:matrix.org'::text))
         Filter: (membership = 'Membership.JOIN'::text)
         Rows Removed by Filter: 11
         Heap Blocks: exact=11
         ->  BitmapAnd  (cost=452.71..452.71 rows=1 width=0) (actual time=3.752..3.753 rows=0 loops=1)
               ->  Bitmap Index Scan on room_memberships_user_id  (cost=0.00..83.26 rows=3482 width=0) (actual time=1.6
03..1.603 rows=545 loops=1)
                     Index Cond: (user_id = ANY ('{@redacted:matrix.org,@redacted2:matrix.org}'::text[]))
               ->  Bitmap Index Scan on room_memberships_room_id  (cost=0.00..369.20 rows=16350 width=0) (actual time=2
.077..2.077 rows=683 loops=1)
                     Index Cond: (room_id = 'redacted:matrix.org'::text)
   ->  Index Scan using current_state_events_room_id_type_state_key_key on current_state_events c  (cost=0.69..4.71 row
s=1 width=89) (never executed)
         Index Cond: ((room_id = 'redacted:matrix.org'::text) AND (type = 'm.room.member'::text) AND (state_
key = m.user_id))
         Filter: (m.event_id = event_id)
 Planning time: 1.850 ms
 Execution time: 5.217 ms

Query plan after the proposed change:

Nested Loop  (cost=1.39..12.15 rows=1 width=78) (actual time=0.063..0.064 rows=0 loops=1)
   ->  Index Scan using current_state_events_room_id_type_state_key_key on current_state_events c  (cost=0.69..7.41 row
s=1 width=89) (actual time=0.030..0.039 rows=1 loops=1)
         Index Cond: ((room_id = '!redacted:matrix.org'::text) AND (type = 'm.room.member'::text) AND (state_
key = ANY ('{@redacted:matrix.org,@redacted2:matrix.org}'::text[])))
   ->  Index Scan using room_memberships_event_id_key on room_memberships m  (cost=0.70..4.72 rows=1 width=156) (actual
 time=0.021..0.021 rows=0 loops=1)
         Index Cond: (event_id = c.event_id)
         Filter: ((room_id = 'redacted:matrix.org'::text) AND (membership = 'Membership.JOIN'::text) AND (c.
state_key = user_id))
         Rows Removed by Filter: 1
 Planning time: 1.578 ms
 Execution time: 0.106 ms

[H-Shay]

Re: timing, I was seeing in Jaeger traces that the query was taking 1-2 seconds on average pretty consistently.

[DMRobertson]

Thanks for indulging me. Looks great!

[H-Shay]

Merged, thanks!