-- Keep a mission's events past the global retention window.
--
-- `mission_gc` deletes mission_events older than EVENT_RETENTION_DAYS (7). That
-- bound is right for volume — a single busy coding phase adds hundreds of rows
-- — and wrong for anything that needs to be re-read later: a Skill-Use
-- measurement, a provenance question, an incident review. All three ask about a
-- specific mission, so the exemption is per-mission rather than a raised global.
--
-- NULL (the default) means the mission obeys the global window, so this changes
-- nothing for existing rows.
ALTER TABLE missions
    ADD COLUMN IF NOT EXISTS retain_events_until TIMESTAMPTZ;

COMMENT ON COLUMN missions.retain_events_until IS
    'While in the future, mission_gc will not reap this mission''s events. Set when a mission is under measurement or investigation.';

-- The sweep joins on this, and the vast majority of rows are NULL.
CREATE INDEX IF NOT EXISTS missions_retain_events_idx
    ON missions (retain_events_until)
    WHERE retain_events_until IS NOT NULL;