-- Marks a track whose file the scanner could no longer find (#2523).
--
-- NULL means present. A timestamp means the file was absent as of that scan,
-- and is the point from which "how long has this been gone" is measured — which
-- is what a later cleanup pass needs in order to require a grace period rather
-- than deleting on a single missed stat.
--
-- Deliberately a nullable timestamp rather than a boolean: "missing" is not a
-- state we want to act on immediately, and the age is the only thing that makes
-- an automated deletion safe to reason about.
--
-- No default and no backfill. Existing rows start NULL (present) and the next
-- full scan sets the mark where it belongs — a migration cannot check the
-- filesystem, and guessing here would mark the whole library on a server whose
-- media volume happens to be detached at upgrade time.
ALTER TABLE tracks
    ADD COLUMN missing_since timestamptz;

-- Partial index: the only query that filters on this column positively is the
-- admin "what's missing" list, which is a small set. Playback and browse
-- queries filter `missing_since IS NULL`, which matches nearly every row and is
-- better served by a sequential scan than an index lookup.
CREATE INDEX tracks_missing_since_idx
    ON tracks (missing_since)
    WHERE missing_since IS NOT NULL;