Re: numeric precision when raising one numeric to another.

Hmmm....
I underestimated.

pow(99999.99999,99999.99999) =
   9.998748785736894607828527462269893046126336085
91664915498635306081273911645075964079222720857427
35641018572673827935330501923067157794798212338823
24997145234949798725508071849154834025252682619864
09675931105114160107573542813573334036043627693673
32584230414090115274301822704676399594689777183090
95124350838052746795283582659784697437868624515447
84308955024802754764364277858847454870139679632204
93566098207186651878539285222697852739872657689082
77740528466769263852694444704577829403518386946691
11157539964528436618742040945886361696712501785143
49612003446329175703756667138162553151705912580792
12331560317684418171064195077598932031644579554853
98595138860229023469055949001949521877405516916475
97554564462253024119778312344592336542732038212175
43130812948451126588746192211036266786198594583755
89036373827433475892132965189682874790600247279436
07120265912512012429492123644988164587146533255393
93335345599658088256314460922495519381049143246081
37075434256493449284197921246089978660147299071527
8174795070535064342859550611e499999

So the precision calculation would be much more complicated.

> -----Original Message-----
> From: Dann Corbit
> Sent: Thursday, May 19, 2005 2:20 PM
> To: 'Martijn van Oosterhout'
> Cc: Alvaro Herrera; John Burger; pgsql-general@postgresql.org
> Subject: RE: [GENERAL] numeric precision when raising one numeric to
> another.
>
> If you want to create a pow() function for numeric using existing
numeric
> functions, it [the new function] should be aware of the precision of
the
> inputs, and the precision of the output should be their product.
>
> So, if you do pow(numeric(10,5), numeric(10,5)) then the result column
> should be numeric(100,25) if you want to retain full precision.
>
> > -----Original Message-----
> > From: Martijn van Oosterhout [mailto:kleptog@svana.org]
> > Sent: Thursday, May 19, 2005 2:02 PM
> > To: Dann Corbit
> > Cc: Alvaro Herrera; John Burger; pgsql-general@postgresql.org
> > Subject: Re: [GENERAL] numeric precision when raising one numeric to
> > another.
> >
> > On Thu, May 19, 2005 at 10:41:51AM -0700, Dann Corbit wrote:
> > > We use Moshier's excellent qfloat numbers.
> > > http://www.moshier.net/qlib.zip
> > > Documentation:
> > > http://www.moshier.net/qlibdoc.html
> > >
> > > So, if you do the following query using CONNX:
> > > select convert(pow(9.5,5.9), varchar)
> > > You will get:
> > >
>
586906.97548405202106027547827738573075504470845684721318303336760202394
> > > 5916438064873363100477233500417619
> >
> > But it's not accurate enough with the default settings. For example
> > 2^100:
> >
> > # select exp( ln(2::numeric) * 100 );
> >                        exp
> > --------------------------------------------------
> >  1267650600228229400579922894637.9015824515440063
> > (1 row)
> >
> > The answer should be:
> >  1267650600228229401496703205376
> >
> > So it's wrong from the 14th digit onwards. If that's the case you
may
> > as well stick to using floating point. It does however appear you
can
> > influence the precision, See:
> >
> > # select exp( ln(2::numeric(50,30)) * 100 );
> >                               exp
> > ----------------------------------------------------------------
> >  1267650600228229401496703205375.991370405139384131115870698781
> > (1 row)
> >
> > Using numeric(50,25) gets you only 28 correct digits. So, if you
know
> > how big your result is going to be you can adjust the types to match
> > and get whatever precision you want. Given that you can estimate the
> > number of digits easily enough (it's linear with the value before
the
> > exp()) maybe you can get it to automatically choose the right
> > precision?
> > --
> > Martijn van Oosterhout   <kleptog@svana.org>
http://svana.org/kleptog/
> > > Patent. n. Genius is 5% inspiration and 95% perspiration. A patent
is
> a
> > > tool for doing 5% of the work and then sitting around waiting for
> > someone
> > > else to do the other 95% so you can sue them.