BUG #2117: inconsistency in sum

On Thu, Dec 15, 2005 at 11:03:55AM +0000, Adam Kolany wrote:

can you explain this?
======================================================
stoff=> select sum(wplyw),sum(wydatek), sum(wplyw)-sum(wydatek) from
rachunki where okres<6;
sum | sum | ?column?
-------+---------+----------
73745 | 6712.55 | 67032.5
(1 row)

What data types are wplyw and wydatek?

--
Michael Fuhr

"Adam Kolany" <dr.a.kolany@wp.pl> writes:

can you explain this?
======================================================
stoff=> select sum(wplyw),sum(wydatek), sum(wplyw)-sum(wydatek) from
rachunki where okres<6;
sum | sum | ?column?
-------+---------+----------
73745 | 6712.55 | 67032.5
(1 row)

Seems odd, but you haven't provided nearly enough information to let
anyone else reproduce the problem.

regards, tom lane

[Please copy the mailing list on replies.]

On Thu, Dec 15, 2005 at 07:32:54PM +0100, Adam Kolany wrote:

Michael Fuhr napisa?(a):

On Thu, Dec 15, 2005 at 11:03:55AM +0000, Adam Kolany wrote:

stoff=> select sum(wplyw),sum(wydatek), sum(wplyw)-sum(wydatek) from
rachunki where okres<6;
sum | sum | ?column?
-------+---------+----------
73745 | 6712.55 | 67032.5
(1 row)

What data types are wplyw and wydatek?

they were float(2)

casting them into numeric helped, so I have changed the types of the to
numeric, instead of float(2)

this is however a bug, I think.

float(2) gives you a real, aka float4 (32-bit floating point). As
the documentation points out, that type has a precision of 6 decimal
digits; you seem to object to the rounding:

test=> SELECT 73745::float8 - 6712.55::float8;
?column?
----------
67032.45
(1 row)

test=> SELECT 67032.45::float4;
float4
---------
67032.5
(1 row)

test=> SELECT 73745::float4 - 6712.55::float4;
?column?
----------
67032.5
(1 row)

That's arguably not a bug: you've requested a low-precision data
type so you have to expect discrepancies around that 6th digit of
precision. If you need greater precision then use a double precision
(float8) type, or if you need exact precision (e.g., for handling
money) then use numeric.

--
Michael Fuhr