Re: [COMMITTERS] pgsql: Fix NUMERIC modulus to properly

I don't think we can justify having NUMERIC division default to
truncation, especially since most division has non-zero decimal digits.

---------------------------------------------------------------------------

Paul Tillotson wrote:
> Bruce Momjian wrote:
>
> >Tom Lane wrote:
> >
> >
> >>Christopher Kings-Lynne <chriskl(at)familyhealth(dot)com(dot)au> writes:
> >>
> >>
> >>>>No, I don't think so. It doesn't seem to be something that enough
> >>>>people use to risk the change in behavior --- it might break something
> >>>>that was working. But, if folks want it backported we can do it. It is
> >>>>only a change to properly do modulus for numeric.
> >>>>
> >>>>
> >>>Well, from my point of view it's an absolute mathematical error - i'd
> >>>backport it. I can't see anyone relying on it :)
> >>>
> >>>
> >>Doesn't this patch break the basic theorem that
> >>
> >> a = trunc(a / b) * b + (a mod b)
> >>
> >>? If division rounds and mod doesn't, you've got pretty serious issues.
> >>
> >>
> >
> >Well, this is a good question. In the equation above we assume '/' is
> >an integer division. The problem with NUMERIC when used with zero-scale
> >operands is that the result is already _rounded_ to the nearest hole
> >number before it gets to trunc(), and that is why we used to get
> >negative modulus values. I assume the big point is that we don't offer
> >any way for users to get a NUMERIC division without rounding.
> >
> >With integers, we always round down to the nearest whole number on
> >division; float doesn't offer a modulus operator, and C doesn't support
> >it either.
> >
> >We round NUMERICs to the specific scale because we want to give the most
> >accurate value:
> >
> > test=> select 100000000000000000000000::numeric(24,0) /
> > 11::numeric(24,0);
> > ?column?
> > ------------------------
> > 9090909090909090909091
> >
> >The actual values is:
> > --
> > 9090909090909090909090.90
> >
> >But the problem is that the equation at the top assumes the division is
> >not rounded. Should we supply a NUMERIC division operator that doesn't
> >round? integer doesn't need it, and float doesn't have the accurate
> >precision needed for modulus operators. The user could supply some
> >digits in the division:
> >
> > test=> select 100000000000000000000000::numeric(30,6) /
> > 11::numeric(24,0);
> > ?column?
> > -------------------------------
> > 9090909090909090909090.909091
> > (1 row)
> >
> >but there really is no _right_ value to prevent rounding (think
> >0.9999999). A non-rounding NUMERIC division would require duplicating
> >numeric_div() but with a false for 'round', and adding operators.
> >
> >
> >
> I would prefer that division didn't round, as with integers. You can
> always calculate your result to 1 more decimal place and then round, but
> there is no way to unround a rounded result.
>
> Tom had asked whether PG passed the regression tests if we change the
> round_var() to a trunc_var() at the end of the function div_var().
>
> It does not pass, but I think that is because the regression test is
> expecting that division will round up. (Curiously, the regression test
> for "numeric" passes, but the regression test for aggregation--sum() I
> think--is the one that fails.) I have attached the diffs here if anyone
> is interested.
>
> Regards,
> Paul Tillotson
>

> *** ./expected/aggregates.out Sun May 29 19:58:43 2005
> --- ./results/aggregates.out Mon Jun 6 21:01:11 2005
> ***************
> *** 10,16 ****
> SELECT avg(a) AS avg_32 FROM aggtest WHERE a < 100;
> avg_32
> ---------------------
> ! 32.6666666666666667
> (1 row)
>
> -- In 7.1, avg(float4) is computed using float8 arithmetic.
> --- 10,16 ----
> SELECT avg(a) AS avg_32 FROM aggtest WHERE a < 100;
> avg_32
> ---------------------
> ! 32.6666666666666666
> (1 row)
>
> -- In 7.1, avg(float4) is computed using float8 arithmetic.
>
> ======================================================================
>

> test boolean ... ok
> test char ... ok
> test name ... ok
> test varchar ... ok
> test text ... ok
> test int2 ... ok
> test int4 ... ok
> test int8 ... ok
> test oid ... ok
> test float4 ... ok
> test float8 ... ok
> test bit ... ok
> test numeric ... ok
> test strings ... ok
> test numerology ... ok
> test point ... ok
> test lseg ... ok
> test box ... ok
> test path ... ok
> test polygon ... ok
> test circle ... ok
> test date ... ok
> test time ... ok
> test timetz ... ok
> test timestamp ... ok
> test timestamptz ... ok
> test interval ... ok
> test abstime ... ok
> test reltime ... ok
> test tinterval ... ok
> test inet ... ok
> test comments ... ok
> test oidjoins ... ok
> test type_sanity ... ok
> test opr_sanity ... ok
> test geometry ... ok
> test horology ... ok
> test insert ... ok
> test create_function_1 ... ok
> test create_type ... ok
> test create_table ... ok
> test create_function_2 ... ok
> test copy ... ok
> test constraints ... ok
> test triggers ... ok
> test create_misc ... ok
> test create_aggregate ... ok
> test create_operator ... ok
> test create_index ... ok
> test inherit ... ok
> test vacuum ... ok
> test create_view ... ok
> test sanity_check ... ok
> test errors ... ok
> test select ... ok
> test select_into ... ok
> test select_distinct ... ok
> test select_distinct_on ... ok
> test select_implicit ... ok
> test select_having ... ok
> test subselect ... ok
> test union ... ok
> test case ... ok
> test join ... ok
> test aggregates ... FAILED
> test transactions ... ok
> test random ... ok
> test portals ... ok
> test arrays ... ok
> test btree_index ... ok
> test hash_index ... ok
> test update ... ok
> test namespace ... ok
> test privileges ... ok
> test misc ... ok
> test select_views ... ok
> test portals_p2 ... ok
> test rules ... ok
> test foreign_key ... ok
> test cluster ... ok
> test limit ... ok
> test plpgsql ... ok
> test copy2 ... ok
> test temp ... ok
> test domain ... ok
> test rangefuncs ... ok
> test prepare ... ok
> test without_oid ... ok
> test conversion ... ok
> test truncate ... ok
> test alter_table ... ok
> test sequence ... ok
> test polymorphism ... ok
> test rowtypes ... ok
> test stats ... ok
> test tablespace ... ok

>
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