Fix query for room participation (#18345)

Follow on from #18068

Currently the subquery in `UPDATE` is pointless, as it will still just
update all `room_membership` rows. Instead, we should look at the
current membership event ID (which is easily retrieved from
`local_current_membership`). We also add a `AND NOT participant` to noop
the `UPDATE` when the `participant` flag is already set.

cc @H-Shay

changelog.d/18345.bugfix

@@ -0,0 +1 @@
+Fix minor performance regression caused by tracking of room participation. Regressed in v1.128.0.

synapse/storage/databases/main/roommember.py

@@ -1622,14 +1622,11 @@ class RoomMemberWorkerStore(EventsWorkerStore, CacheInvalidationWorkerStore):
             sql = """
                 UPDATE room_memberships
                 SET participant = true
-                WHERE (user_id, room_id) IN (
+                WHERE event_id IN (
-                    SELECT user_id, room_id
+                    SELECT event_id FROM local_current_membership
-                    FROM room_memberships
+                    WHERE user_id = ? AND room_id = ?
-                    WHERE user_id = ?
-                    AND room_id = ?
-                    ORDER BY event_stream_ordering DESC
-                    LIMIT 1
                 )
+                AND NOT participant
             """
             txn.execute(sql, (user_id, room_id))
 
@@ -1651,11 +1648,10 @@ class RoomMemberWorkerStore(EventsWorkerStore, CacheInvalidationWorkerStore):
         ) -> bool:
             sql = """
                 SELECT participant
-                FROM room_memberships
+                FROM local_current_membership AS l
-                WHERE user_id = ?
+                INNER JOIN room_memberships AS r USING (event_id)
-                AND room_id = ?
+                WHERE l.user_id = ?
-                ORDER BY event_stream_ordering DESC
+                AND l.room_id = ?
-                LIMIT 1
             """
             txn.execute(sql, (user_id, room_id))
             res = txn.fetchone()