Thread: How many digits are printed with double precision?

Hello,

I'm using postgreSQL and python (PyGreSQL) and face a strange problem.

I have a double precision column in my DB (it's the time at which an image was taken expressed in second from UNIX epoch). When I query the value (SELECT time...) I get only 5 digits after the dot (while I suppose there are 6 digits). I have the same result when using pgadmin. The results looks like the rounding of the desired time (which I know by the filename).

Is it because postgreSQL only returns a fixed amount of digits or is the mistake is coming from my INSERTion (something like a semiconscious rounding).

Thanks in advance,
Mathieu

Mathieu Dubois <mathieu.dubois@limsi.fr> writes:
> I have a double precision column in my DB (it's the time at which an
> image was taken expressed in second from UNIX epoch). When I query the
> value (SELECT time...) I get only 5 digits after the dot (while I
> /suppose/ there are 6 digits). I have the same result when using
> pgadmin. The results looks like the rounding of the desired time (which
> I know by the filename).

See extra_float_digits.  Be aware that cranking that up produces digits
that are not 100% trustworthy.  If you are expecting exact results of
more than 15 places, you likely shouldn't be using float arithmetic.

            regards, tom lane

Hi Tom,

Thanks for the very fast answer but what do you mean exactly by :

"See extra_float_digits."

I have found messages on the mailing list but it doesn't help (I don't
want to recompile everything). Is there an option in some configuration
file to set?

Mathieu
On 03/23/2011 10:02 PM, Tom Lane wrote:
> Mathieu Dubois<mathieu.dubois@limsi.fr>  writes:
>> I have a double precision column in my DB (it's the time at which an
>> image was taken expressed in second from UNIX epoch). When I query the
>> value (SELECT time...) I get only 5 digits after the dot (while I
>> /suppose/ there are 6 digits). I have the same result when using
>> pgadmin. The results looks like the rounding of the desired time (which
>> I know by the filename).
> See extra_float_digits.  Be aware that cranking that up produces digits
> that are not 100% trustworthy.  If you are expecting exact results of
> more than 15 places, you likely shouldn't be using float arithmetic.
>
>             regards, tom lane
>

http://lmgtfy.com/?q=postgresql+documentation+extra_float_digits


On 03/23/11 17:24, Mathieu Dubois wrote:
> Hi Tom,
>
> Thanks for the very fast answer but what do you mean exactly by :
>
> "See extra_float_digits."
>
> I have found messages on the mailing list but it doesn't help (I don't
> want to recompile everything). Is there an option in some configuration
> file to set?
>
> Mathieu
> On 03/23/2011 10:02 PM, Tom Lane wrote:
>> Mathieu Dubois<mathieu.dubois@limsi.fr> writes:
>>> I have a double precision column in my DB (it's the time at which an
>>> image was taken expressed in second from UNIX epoch). When I query the
>>> value (SELECT time...) I get only 5 digits after the dot (while I
>>> /suppose/ there are 6 digits). I have the same result when using
>>> pgadmin. The results looks like the rounding of the desired time (which
>>> I know by the filename).
>> See extra_float_digits. Be aware that cranking that up produces digits
>> that are not 100% trustworthy. If you are expecting exact results of
>> more than 15 places, you likely shouldn't be using float arithmetic.
>>
>> regards, tom lane

OK, I finally found it. Thanks I will try to set the value to 1 or 2 for
the moment.

On 03/23/2011 10:35 PM, Frank Bax wrote:
>
> http://lmgtfy.com/?q=postgresql+documentation+extra_float_digits
>
>
> On 03/23/11 17:24, Mathieu Dubois wrote:
>> Hi Tom,
>>
>> Thanks for the very fast answer but what do you mean exactly by :
>>
>> "See extra_float_digits."
>>
>> I have found messages on the mailing list but it doesn't help (I don't
>> want to recompile everything). Is there an option in some configuration
>> file to set?
>>
>> Mathieu
>> On 03/23/2011 10:02 PM, Tom Lane wrote:
>>> Mathieu Dubois<mathieu.dubois@limsi.fr> writes:
>>>> I have a double precision column in my DB (it's the time at which an
>>>> image was taken expressed in second from UNIX epoch). When I query the
>>>> value (SELECT time...) I get only 5 digits after the dot (while I
>>>> /suppose/ there are 6 digits). I have the same result when using
>>>> pgadmin. The results looks like the rounding of the desired time
>>>> (which
>>>> I know by the filename).
>>> See extra_float_digits. Be aware that cranking that up produces digits
>>> that are not 100% trustworthy. If you are expecting exact results of
>>> more than 15 places, you likely shouldn't be using float arithmetic.
>>>
>>> regards, tom lane
>