BEGIN;

-- A monotonic, server-assigned position for every visit, so a live feed can be
-- paged without losing anyone.
--
-- The arrivals feed originally ordered by (occurred_at, id). That is wrong in a
-- way that only appears under the exact condition the feed exists for: several
-- people walking through one door together share an occurred_at to the
-- microsecond, so the tiebreaker was a RANDOM uuid. A visit committed after the
-- reader had moved its cursor, but carrying a lower uuid, sorted behind the
-- cursor and was never delivered - a silent footfall undercount, exactly the
-- class of bug this system is otherwise careful about. Measured live: four
-- simultaneous visits, two delivered.
--
-- occurred_at cannot fix it either. It is the CAMERA's clock, and a site that
-- was offline for a day floods in with yesterday's timestamps; a reader whose
-- cursor is already past them would skip the entire backlog.
--
-- So the feed is ordered by when the SERVER learned of a visit, not by when it
-- happened. Each row still carries occurred_at for display; seq is only ever a
-- position. That is what makes a reconnecting site's backlog get delivered
-- rather than hidden behind a timestamp the reader has passed.
ALTER TABLE visits ADD COLUMN IF NOT EXISTS seq bigserial;

-- The feed always filters by tenant and orders by seq, so this is the index it
-- runs on. Without it every poll is a scan of the whole table.
CREATE INDEX IF NOT EXISTS visits_client_seq_idx ON visits (client_id, seq);
CREATE INDEX IF NOT EXISTS visits_site_seq_idx   ON visits (site_id, seq);

-- Not UNIQUE by accident: bigserial already guarantees it, and the constraint
-- would make a future partitioning change harder for no gain.

COMMIT;