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?
---------------------------------------------------------------------------------------
18446744073709551616.0000000000000000000000000000000000000000000000000000000000000000
(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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