code: func (b *builtinExtractDurationSig) evalInt(row chunk.Row) (int64, bool, error) {
  	unit, isNull, err := b.args[0].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return 0, isNull, err
  	}
  	dur, isNull, err := b.args[1].EvalDuration(b.ctx, row)
  	if isNull || err != nil {
  		return 0, isNull, err
  	}
  	res, err := types.ExtractDurationNum(&dur, unit)
  	return res, err != nil, err
  }
docs: // evalInt evals a builtinExtractDurationSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_extract