code: func ComputeDiv(sc *variable.StatementContext, a, b Datum) (d Datum, err error) {
  	// MySQL support integer division Div and division operator /
  	// we use opcode.Div for division operator and will use another for integer division later.
  	// for division operator, we will use float64 for calculation.
  	switch a.Kind() {
  	case KindFloat64:
  		y, err1 := b.ToFloat64(sc)
  		if err1 != nil {
  			return d, errors.Trace(err1)
  		}
  
  		if y == 0 {
  			return d, nil
  		}
  
  		x := a.GetFloat64()
  		d.SetFloat64(x / y)
  		return d, nil
  	default:
  		// the scale of the result is the scale of the first operand plus
  		// the value of the div_precision_increment system variable (which is 4 by default)
  		// we will use 4 here
  		xa, err1 := a.ToDecimal(sc)
  		if err != nil {
  			return d, errors.Trace(err1)
  		}
  
  		xb, err1 := b.ToDecimal(sc)
  		if err1 != nil {
  			return d, errors.Trace(err1)
  		}
  		// division by zero return null
  		to := new(MyDecimal)
  		err = DecimalDiv(xa, xb, to, DivFracIncr)
  		if err != ErrDivByZero {
  			d.SetMysqlDecimal(to)
  		} else {
  			err = nil
  		}
  		return d, err
  	}
  }
docs: // ComputeDiv computes the result of a/b.