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

With no conclusion on this, I have added a TODO item:

* Add NUMERIC division operator that doesn't round?

Currently NUMERIC _rounds_ the result to the specified precision.
This means division can return a result that multiplied by the
divisor is greater than the dividend, e.g. this returns a value > 10:

SELECT (10::numeric(2,0) / 6::numeric(2,0))::numeric(2,0) * 6;

The positive modulus result returned by NUMERICs might be considered
inaccurate, in one sense.

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

Bruce Momjian wrote:
>
> Have we made any decision on whether the old/new NUMERIC %(mod) code was
> correct, and now to handle rounding? Should we offer a non-rounding
> division operator for NUMERIC?
>
> ---------------------------------------------------------------------------
>
> 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
>
> --
> Bruce Momjian | http://candle.pha.pa.us
> pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 359-1001
> + If your life is a hard drive, | 13 Roberts Road
> + Christ can be your backup. | Newtown Square, Pennsylvania 19073
>
> ---------------------------(end of broadcast)---------------------------
> TIP 7: don't forget to increase your free space map settings
>

--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073