code: func (m *Meta) GetDDLJobByIdx(index int64, jobListKeys ...JobListKeyType) (*model.Job, error) {
  	listKey := m.jobListKey
  	if len(jobListKeys) != 0 {
  		listKey = jobListKeys[0]
  	}
  
  	startTime := time.Now()
  	job, err := m.getDDLJob(listKey, index)
  	metrics.MetaHistogram.WithLabelValues(metrics.GetDDLJobByIdx, metrics.RetLabel(err)).Observe(time.Since(startTime).Seconds())
  	return job, errors.Trace(err)
  }
docs: // GetDDLJobByIdx returns the corresponding DDL job by the index.
  // The length of jobListKeys can only be 1 or 0.
  // If its length is 1, we need to replace m.jobListKey with jobListKeys[0].
  // Otherwise, we use m.jobListKey directly.