ALTER TABLE mutations
    ADD COLUMN IF NOT EXISTS dispatch_started_at TIMESTAMPTZ,
    ADD COLUMN IF NOT EXISTS request_deadline_at TIMESTAMPTZ,
    ADD COLUMN IF NOT EXISTS worker_lease_until TIMESTAMPTZ,
    ADD COLUMN IF NOT EXISTS settled_at TIMESTAMPTZ;

DO $$
BEGIN
    IF EXISTS (
        SELECT 1
        FROM mutations
        WHERE external_state = 'pending'
    ) THEN
        RAISE EXCEPTION
            'recovery settlement migration requires no pending mutations';
    END IF;
END
$$;

ALTER TABLE mutations
    ADD CONSTRAINT mutations_dispatch_timing_check CHECK (
        (external_state <> 'pending') OR
        (dispatch_started_at IS NOT NULL AND
         request_deadline_at IS NOT NULL AND
         worker_lease_until IS NOT NULL)
    ) NOT VALID;

ALTER TABLE mutations
    VALIDATE CONSTRAINT mutations_dispatch_timing_check;

CREATE INDEX IF NOT EXISTS mutations_recovery_idx
    ON mutations (worker_lease_until, request_deadline_at, updated_at)
    WHERE state IN ('prepared', 'applying');