Re: [GENERAL] Floating point error

From: Albe Laurenz <laurenz(dot)albe(at)wien(dot)gv(dot)at>
To: "Maciek Sakrejda *EXTERN*" <m(dot)sakrejda(at)gmail(dot)com>
Cc: Daniel Farina *EXTERN* <daniel(at)heroku(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Tom Duffey *EXTERN* <tduffey(at)trillitech(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [GENERAL] Floating point error
Date: 2013-03-06 10:04:25
Message-ID: A737B7A37273E048B164557ADEF4A58B057BB09A@ntex2010a.host.magwien.gv.at
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Maciek Sakrejda wrote:
> On Tue, Mar 5, 2013 at 12:03 AM, Albe Laurenz <laurenz(dot)albe(at)wien(dot)gv(dot)at> wrote:
>> I don't think that it is about looking nice.
>> C doesn't promise you more than FLT_DIG or DBL_DIG digits of
>> precision, so PostgreSQL cannot either.
>>
>> If you allow more, that would mean that if you store the same
>> number on different platforms and query it, it might come out
>> differently. Among other things, that would be a problem for
>> the regression tests.
>
> Thank you: I think this is what I was missing, and what wasn't clear
> from the proposed doc patch. But then how can pg_dump assume that it's
> always safe to set extra_float_digits = 3? Why the discrepancy between
> default behavior and what pg_dump gets? It can't know whether the dump
> is to be restored into the same system or a different one (and AFAICT,
> there's not even an option to tweak extra_float_digits there).

How about this elaboration?

Yours,
Laurenz Albe

Attachment Content-Type Size
float-2.patch application/octet-stream 1.6 KB

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