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Merge pull request #441 from matrix-org/s7evink/startup-perf

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Optimize getting the latest events by type per room
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Kegan Dougal 2024-05-21 18:13:06 +01:00 committed by GitHub
commit 98b12cba6a
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1 changed files with 23 additions and 12 deletions
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35
state/event_table.go
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@ -441,18 +441,29 @@ func (t *EventTable) SelectLatestEventsBetween(txn *sqlx.Tx, roomID string, lowe
func (t *EventTable) selectLatestEventByTypeInAllRooms(txn *sqlx.Tx) ([]Event, error) {
result := []Event{}
// TODO: this query ends up doing a sequential scan on the events table. We have
// an index on (event_type, room_id, event_nid) so I'm a little surprised that PG
// decides to do so. Can we do something better here? Ideas:
// - Find a better query for selecting the newest event of each type in a room.
// - At present we only care about the _timestamps_ of these events. Perhaps we
// could store those in the DB (and even in an index) as a column and select
// those, to avoid having to parse the event bodies.
// - We could have the application maintain a `latest_events` table so that the
// rows can be directly read. Assuming a mostly-static set of event types, reads
// are then linear in the number of rooms.
rows, err := txn.Query(
`SELECT room_id, event_nid, event FROM syncv3_events WHERE event_nid in (SELECT MAX(event_nid) FROM syncv3_events GROUP BY room_id, event_type)`,
// What the following query does:
// 1. Gets all event types from a recursive CTE as the `event_types` CTE
// 2. Gets all rooms as the `room_ids` CTE
// 3. Gets the latest event_nid for each event_type and room as the `max_by_ev_type` CTE
// 4. Queries the required data using the event_nids provided by the `max_by_ev_type` CTE
rows, err := txn.Query(`
WITH event_types AS (
WITH RECURSIVE t AS (
(SELECT event_type FROM syncv3_events ORDER BY event_type LIMIT 1) -- parentheses required
UNION ALL
SELECT (SELECT event_type FROM syncv3_events WHERE event_type > t.event_type ORDER BY event_type LIMIT 1)
FROM t
WHERE t.event_type IS NOT NULL
)
SELECT event_type FROM t WHERE event_type IS NOT NULL
), room_ids AS (
SELECT DISTINCT room_id FROM syncv3_rooms
), max_by_ev_type AS (
SELECT m.max FROM event_types, room_ids,
LATERAL ( SELECT max(event_nid) as max FROM syncv3_events e WHERE e.room_id = room_ids.room_id AND e.event_type = event_types.event_type ) AS m
)
SELECT room_id, event_nid, event FROM syncv3_events, max_by_ev_type WHERE event_nid = max_by_ev_type.max
`,
)
if err != nil {
return nil, err