-- Delivery state is separate from visibility. A scheduled alert becomes visible
-- at start_time; this state makes sure its push notification is sent once.
ALTER TABLE alerts
    ADD COLUMN IF NOT EXISTS notification_sent_at TIMESTAMPTZ,
    ADD COLUMN IF NOT EXISTS notification_claimed_at TIMESTAMPTZ,
    ADD COLUMN IF NOT EXISTS notification_attempts INTEGER NOT NULL DEFAULT 0;

CREATE INDEX IF NOT EXISTS idx_alerts_pending_notification
    ON alerts (start_time)
    WHERE is_active = true AND notification_sent_at IS NULL;

-- Earlier versions sent every active alert immediately on creation. Mark those
-- already-created future alerts as dispatched, so upgrading does not notify
-- passengers a second time when the scheduler starts.
UPDATE alerts
SET notification_sent_at = created_at
WHERE notification_sent_at IS NULL
  AND is_active = true
  AND start_time > created_at;