maria-developers team mailing list archive

[Alexander Barkov <bar@xxxxxxxxxxx>]

Hello Sergei,


On 07/23/2014 08:50 PM, Sergei Golubchik wrote:
> Hi, Alexander!
>
> > I removed find_num_type() and fix_num_length_and_dec() which
> > were hard to track, and for some Items these methods were
> > erroneously called twice.
> >
> > fix_num_length_and_dec() now only exists as a non-virtual method in
> > Item_sum and Item_udf_func.
>
> Why did you keep them?


Thanks for noticing this.

It seems the only thing that Item_sum::fix_length_and_dec() and 
Item_udf_func::fix_length_and_dec() do is a call for
fix_num_length_and_dec().

I will remove them.


> > === modified file 'sql/item_func.cc'
> > --- sql/item_func.cc	2014-06-04 16:32:57 +0000
> > +++ sql/item_func.cc	2014-06-09 07:58:55 +0000
> > @@ -1618,8 +1606,20 @@ my_decimal *Item_func_div::decimal_op(my
> >
> >   void Item_func_div::result_precision()
> >   {
> > +  /*
> > +    We need to add args[1]->divisor_precision_increment(),
> > +    to properly handle the cases like this:
> > +      SELECT 5.05 / 0.014; -> 360.714286
> > +    i.e. when the divisor has a zero integer part
> > +    and non-zero digits appear only after the decimal point.
> > +    Precision in this example is calculated as
> > +      args[0]->decimal_precision()           +  // 3
> > +      args[1]->divisor_precision_increment() +  // 3
> > +      prec_increment                            // 4
> > +    which gives 10 decimals digits.
> > +  */
> >     uint precision=min(args[0]->decimal_precision() +
> > -                     args[1]->decimals + prec_increment,
> > +                     args[1]->divisor_precision_increment() + prec_increment,
>
> I don't understand that, why divisor_precision_increment() and not
> decimals (or decimal_scale()) ?

For two reasons:

1. The implementation is slightly more complex than just
returning "decimals". See item.h:

  uint divisor_precision_increment() const
  {
    return decimals >= NOT_FIXED_DEC &&
           is_temporal_type_with_time(field_type()) ?
           TIME_SECOND_PART_DIGITS : decimals;
  }

and it's slightly different from decimal_scale().




2. See the comment in item.h, near the method declaration.
In the future we can make it virtual to get smaller data types:
The items that never have zero integer part do not add digits
into division results.


> >                        DECIMAL_MAX_PRECISION);
> >
> >     /* Integer operations keep unsigned_flag if one of arguments is unsigned */
>
> Regards,
> Sergei