Does NUMERIC lose precision?

On Mon, May 29, 2017 at 4:19 PM, Kaiting Chen <ktchen14@gmail.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...

--
Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general

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.

Kaiting Chen <ktchen14@gmail.com> writes:

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:

Division is inherently inexact, except in special cases. If you think
that 1/(2^64) should be carried out to enough digits to be exact,
what would you have us do with 1/3?

The actual behavior is that it will choose a result scale (number of
digits after the decimal point) that is dependent on the scales of the
input arguments, but not on their precise values. I don't recall the
details beyond that.

regards, tom lane

--
Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general