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Add a materialized view for event_types and use it in an updated query

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Till Faelligen 2024-05-21 15:27:15 +02:00
parent f25087d56a
commit 9eee30b152
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2 changed files with 31 additions and 13 deletions
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33
state/event_table.go
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@ -141,6 +141,11 @@ func NewEventTable(db *sqlx.DB) *EventTable {
CREATE INDEX IF NOT EXISTS syncv3_nid_room_state_idx ON syncv3_events(room_id, event_nid, is_state);
CREATE UNIQUE INDEX IF NOT EXISTS syncv3_events_room_event_nid_type_skey_idx ON syncv3_events(event_nid, event_type, state_key);
-- Create a materialized view for event_types (used on startup to get the latest events in each room)
CREATE MATERIALIZED VIEW IF NOT EXISTS event_types as
SELECT DISTINCT event_type
FROM syncv3_events;
`)
return &EventTable{db}
}
@ -441,18 +446,22 @@ 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 materialized view (updated on startup in `PrepareSnapshot`) 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 (
SELECT * FROM event_types
), 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

11
state/storage.go
View File

@ -171,7 +171,16 @@ func (s *Storage) PrepareSnapshot(txn *sqlx.Tx) (tableName string, err error) {
`SELECT UNNEST(membership_events) AS membership_nid INTO TEMP ` + tempTableName + ` FROM syncv3_snapshots
JOIN syncv3_rooms ON syncv3_snapshots.snapshot_id = syncv3_rooms.current_snapshot_id`,
)
return tempTableName, err
if err != nil {
return "", err
}
// Refresh the materialized view, so getting latest events by type per room
// can use a fresh view of the event_types.
_, err = txn.Exec("REFRESH MATERIALIZED VIEW event_types;")
if err != nil {
return "", err
}
return tempTableName, nil
}
// GlobalSnapshot snapshots the entire database for the purposes of initialising