code: func (m *Meta) GetSchemaDiff(schemaVersion int64) (*model.SchemaDiff, error) {
  	diffKey := m.schemaDiffKey(schemaVersion)
  	startTime := time.Now()
  	data, err := m.txn.Get(diffKey)
  	metrics.MetaHistogram.WithLabelValues(metrics.GetSchemaDiff, metrics.RetLabel(err)).Observe(time.Since(startTime).Seconds())
  	if err != nil || len(data) == 0 {
  		return nil, errors.Trace(err)
  	}
  	diff := &model.SchemaDiff{}
  	err = json.Unmarshal(data, diff)
  	return diff, errors.Trace(err)
  }
docs: // GetSchemaDiff gets the modification information on a given schema version.