code: func getTargetSchemaAndTable(ctx sessionctx.Context, dbName, tableName string, is infoschema.InfoSchema) (string, table.Table, error) {
  	if len(dbName) == 0 {
  		dbName = ctx.GetSessionVars().CurrentDB
  		if len(dbName) == 0 {
  			return "", nil, errors.New("miss DB name for grant privilege")
  		}
  	}
  	name := model.NewCIStr(tableName)
  	tbl, err := is.TableByName(model.NewCIStr(dbName), name)
  	if err != nil {
  		return "", nil, err
  	}
  	return dbName, tbl, nil
  }
docs: // getTargetSchemaAndTable finds the schema and table by dbName and tableName.