ALTER TABLE duty_rosters
  ADD COLUMN flight_id BINARY(16) NULL AFTER id;

CREATE UNIQUE INDEX idx_roster_flight_id
  ON duty_rosters (flight_id);

-- Keep base/date as a non-unique lookup index so existing roster views still work.
DROP INDEX idx_roster_base_date ON duty_rosters;
CREATE INDEX idx_roster_base_date
  ON duty_rosters (base_id, duty_date);
