code: func builtinDatabase(args []types.Datum, ctx context.Context) (d types.Datum, err error) {
  	currentDB := ctx.GetSessionVars().CurrentDB
  	if currentDB == "" {
  		return d, nil
  	}
  	d.SetString(currentDB)
  	return d, nil
  }
docs: // See https://dev.mysql.com/doc/refman/5.7/en/information-functions.html