-- Partial index backing the true-instant backfill drain (ADR-0001, ticket
-- #118). The drain selects rows still on the legacy convention
-- (`date_utc IS NULL`) per library in id order, and the progress surface
-- (Prometheus `imageserver_date_backfill_remaining` + `/libraries`) counts
-- the same set. Mirrors `idx_image_exif_date_backfill`: the index shrinks
-- to nothing as the drain converges, so it costs ~0 after cutover.
CREATE INDEX idx_image_exif_utc_backfill
    ON image_exif (library_id, id)
    WHERE date_utc IS NULL;