Re: Does NUMERIC lose precision?

On Mon, May 29, 2017 at 4:19 PM, Kaiting Chen <ktchen14(at)gmail(dot)com> wrote:

> Hi everyone. I’m trying to perform some exact precision arithmetic with
> PostgreSQL’s NUMERIC type. However I can’t seem to get the unparameterized
> NUMERIC type to perform exact precision arithmetic:
>
> # SELECT 2::NUMERIC ^ 64;
> ?column?
> ---------------------------------------
> 18446744073709551616.0000000000000000
> (1 row)
>
> While the above operation works fine once I divide 1 by that number the
> result is an inexact decimal number:
>
> # SELECT 1 / (2::NUMERIC ^ 64);
> ?column?
> ----------------------------------------
> 0.000000000000000000054210108624275222
> (1 row)
>
> It doesn't seem to be an issue with the output either as taking the
> reciprocal yields a different number than I started with:
>
> # SELECT 1 / (1 / (2::NUMERIC ^ 64));
> ?column?
> -----------------------------------------------------------
> 18446744073709551514.042092759729171265910020841463748922
> (1 row)
>
> The only way to get an exact result is by specifying an explicit precision
> and scale:
>
> # SELECT 1 / (2::NUMERIC(96, 64) ^ 64);
> ?column?
> --------------------------------------------------------------------
> 0.0000000000000000000542101086242752217003726400434970855712890625
> (1 row)
>
> # SELECT 1 / (1 / (2::NUMERIC(96, 64) ^ 64));
> ?column?
> ------------------------------------------------------------
> ---------------------------
> 18446744073709551616.00000000000000000000000000000000000000
> 00000000000000000000000000
> (1 row)
>
> However this does not seem intuitive from the documentation which states
> that:
>
> Specifying:
>
> NUMERIC
>
> without any precision or scale creates a column in which numeric values of
> any precision and scale can be stored, up to the implementation limit on
> precision. A column of this kind will not coerce input values to any
> particular scale...
>
>
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>While the above operation works fine once I divide 1 by that number the
result is an inexact decimal number:
># SELECT 1 / (2::NUMERIC ^ 64);
? ?column?
>----------------------------------------
>0.000000000000000000054210108624275222
>(1 row)

*That is the same answer you get when you use any calculator. *

*Are you sure you did not meanSELECT 2::NUMERIC^ 64/1;*

*?column?18446744073709551616.0000000000000000*

--
*Melvin Davidson*
I reserve the right to fantasize. Whether or not you
wish to share my fantasy is entirely up to you.