code: func MergeSchema(lSchema, rSchema *Schema) *Schema {
  	if lSchema == nil && rSchema == nil {
  		return nil
  	}
  	if lSchema == nil {
  		return rSchema.Clone()
  	}
  	if rSchema == nil {
  		return lSchema.Clone()
  	}
  	tmpL := lSchema.Clone()
  	tmpR := rSchema.Clone()
  	ret := NewSchema(append(tmpL.Columns, tmpR.Columns...)...)
  	ret.TblID2Handle = tmpL.TblID2Handle
  	for id, cols := range tmpR.TblID2Handle {
  		if _, ok := ret.TblID2Handle[id]; ok {
  			ret.TblID2Handle[id] = append(ret.TblID2Handle[id], cols...)
  		} else {
  			ret.TblID2Handle[id] = cols
  		}
  	}
  	return ret
  }
docs: // MergeSchema will merge two schema into one schema. We shouldn't need to consider unique keys.
  // That will be processed in build_key_info.go.