-- A visit reference a person can use, beside the key a machine uses.
--
-- Every other thing in this product a person refers to already has one: a shop
-- is `chennai`, a camera is `cam1`, a customer is `V-42`, a person is their
-- email. A visit had only its uuid:
--
--     "visit_id": "4cc216ca-dad3-4958-bb96-5f5a82022cf8"
--
-- 012 argued that this was acceptable because no route takes a visit id and
-- nobody says one out loud. That is true of routing and false of everything
-- else: it is what the arrivals feed shows, what a support conversation has to
-- quote, and what somebody reading an API response judges the product by. The
-- owner asked for it twice.
--
-- `#1042`, per client, mirroring `V-42` exactly and for the same three reasons
-- (speakable, per-tenant so it discloses no platform-wide volume, and a
-- reference beside the key rather than a replacement for it - eleven tables
-- reference visits.id).
--
-- Why a stored counter is affordable on the hottest table in the schema:
-- allocating it row-locks the client for the length of one insert, and visits
-- from one tenant are ALREADY serialised - the MQTT consumer sets
-- SetOrderMatters(true) precisely so that `seq` is a commit order. So this
-- adds no contention a tenant did not already have, and tenants never block
-- each other. A derived reference was the alternative and does not work:
-- several people through one door share occurred_at to the microsecond, which
-- is the very collision 004 exists to handle.

ALTER TABLE clients ADD COLUMN IF NOT EXISTS visit_seq bigint NOT NULL DEFAULT 0;
ALTER TABLE visits  ADD COLUMN IF NOT EXISTS number    bigint;

-- Existing rows get their numbers in the order the server learned of them,
-- which is what `seq` means - not occurred_at, which is the camera's clock and
-- arrives out of order after a site has been offline.
WITH numbered AS (
    SELECT id, row_number() OVER (PARTITION BY client_id ORDER BY seq) AS n
      FROM visits
)
UPDATE visits v SET number = numbered.n
  FROM numbered
 WHERE v.id = numbered.id AND v.number IS NULL;

UPDATE clients c
   SET visit_seq = GREATEST(c.visit_seq, COALESCE(
       (SELECT max(number) FROM visits WHERE client_id = c.id), 0));

CREATE UNIQUE INDEX IF NOT EXISTS visits_client_number_idx
    ON visits (client_id, number);

COMMENT ON COLUMN visits.number IS
    'Per-client visit number, shown as #1042. A public reference beside the '
    'uuid key, never a replacement for it.';