-- 0060_play_events_track_index.up.sql — play_events by track (Scribe #3912, #3911).
--
-- play_events is indexed by (user_id, started_at) and (user_id, track_id), both
-- led by user. Nothing reached it by track alone until the duplicates report,
-- which shows each copy's play count — a scan of the whole table per copy — and
-- the merge (#3911), which repoints a duplicate's play history onto the copy
-- being kept. Both ask "every play of this track", whoever played it.
CREATE INDEX play_events_track_idx ON play_events (track_id);