ALTER TABLE tb_outbox_event
    ADD COLUMN parent_event_id varchar(64) NOT NULL DEFAULT '';

CREATE INDEX idx_outbox_event_parent
    ON tb_outbox_event (parent_event_id)
    WHERE parent_event_id <> '';

COMMENT ON TABLE tb_outbox_event IS '公共可靠事件 Outbox';
COMMENT ON COLUMN tb_outbox_event.parent_event_id IS '直接触发该可靠事件的真实审计事件ID，空字符串表示无已落库父事件';