Обсуждение: Bug #644: Incorrect result in math operation

Francisco Blay (franblay@ono.com) reports a bug with a severity of 3
The lower the number the more severe it is.

Short Description
Incorrect result in math operation

Long Description
3088.8 - (2847.64 + 241.16)=4.54747350886464e-13
It's spected to be 0.0 the result of this operation (I think)
Got the same results in

PIII:

franblay=> select version();
                            version
---------------------------------------------------------------
 PostgreSQL 7.0.3 on i686-pc-linux-gnu, compiled by gcc 2.95.2
(1 row)

AMD Athlon:

datadb=# select version();
                           version
-------------------------------------------------------------
 PostgreSQL 7.2 on i686-pc-linux-gnu, compiled by GCC 2.95.2
(1 row)

Both SuSE 7.0
Libraries:
                      libe2p.so.2          libpam.so.0
                      libe2p.so.2.3        libpam.so.0.72
                      libext2fs.so.2       libpam_misc.so.0
                      libext2fs.so.2.4     libpam_misc.so.0.72
ld-2.1.3.so           libhistory.so.4      libpamc.so.0
ld-linux.so.1         libhistory.so.4.1    libpamc.so.0.72
ld-linux.so.1.9.9     libm.so.6            libpthread.so.0
ld-linux.so.2         libncurses.so.4      libreadline.so.4
ld.so                 libncurses.so.4.2    libreadline.so.4.1
ld.so.1.9.9           libncurses.so.5      libresolv.so.2
libBrokenLocale.so.1  libncurses.so.5.0    librt.so.1
libNoVersion.so.1     libnsl.so.1          libss.so.2
libSegFault.so        libnss_compat.so.2   libss.so.2.0
libc.so.6             libnss_db.so.2       libthread_db.so.1
libcom_err.so.2       libnss_dns.so.2      libutil.so.1
libcom_err.so.2.0     libnss_dns6.so.2     libuuid.so.1
libcrypt.so.1         libnss_files.so.2    libuuid.so.1.2
libdb.so.2            libnss_hesiod.so.2   
libdb.so.3            libnss_nis.so.2      
libdl.so.2            libnss_nisplus.so.2


Sample Code
datadb=# select 3088.8 - 3088.8 AS ok;
 ok
----
  0
(1 row)

datadb=# select 3088.8 - (2847.64 + 241.16) AS silly;
        silly
----------------------
 4.54747350886464e-13
(1 row)


No file was uploaded with this report

Hi!

On Wed, 24 Apr 2002 04:33:00 -0400 (EDT)
pgsql-bugs@postgresql.org wrote:

> Francisco Blay (franblay@ono.com) reports a bug with a severity of 3
> The lower the number the more severe it is.
>
> Short Description
> Incorrect result in math operation
>
> Long Description
> 3088.8 - (2847.64 + 241.16)=4.54747350886464e-13
> It's spected to be 0.0 the result of this operation (I think)
> Got the same results in
>
...
> datadb=# select 3088.8 - (2847.64 + 241.16) AS silly;
>         silly
> ----------------------
>  4.54747350886464e-13
> (1 row)
>

I think it's a limitation of floating point numbers representation, not a PostgreSQL bug. Use NUMERIC type, if you need
an"exact" precision: 

org=> SELECT 3088.8::numeric(8,3) - (2847.64::numeric(8,3) + 241.16::numeric(8,3)) AS silly;
 silly
-------
 0.000
(1 row)

Best regards

--
Dmitry Tsitelov
mailto:cit@tag-ltd.spb.ru