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List:       pgsql-bugs
Subject:    Re: BUG #15925: Loss of precision converting money to numeric
From:       Slawomir Chodnicki <slawomir.chodnicki () gmail ! com>
Date:       2019-07-26 16:15:00
Message-ID: A6E2C5F9-7437-41B1-9F18-4E6C35981F6B () gmail ! com
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> Hmm, yeah, anything approaching INT64_MAX has a problem.
> The issue is that cash_numeric() does the equivalent of
> 
> SELECT 9223372036854775807::numeric / 100::numeric;
> 
> and if you try that by hand you indeed get
> 
> 92233720368547758
> 
> because select_div_scale() has decided that it need not produce
> any fractional digits.  We can force its hand by making the input
> have the required number of fractional digits *before* dividing,
> which is a bit weird on its face but gets the job done, per the
> comment therein:
> 
>     * The result scale of a division isn't specified in any SQL standard. For
>     * PostgreSQL we select a result scale that will give at least
>     * NUMERIC_MIN_SIG_DIGITS significant digits, so that numeric gives a
>     * result no less accurate than float8; but use a scale not less than
>     * either input's display scale.
> 
> (NUMERIC_MIN_SIG_DIGITS is 16, whence the problem for a 17-digit result.
> Maybe we should consider raising that, but I'm hesitant to consider such
> a far-reaching change just to make cash_numeric happy.)
> 
> I intend to apply the attached patch.

Thanks Tom,

the response is illuminating. And a same-day patch is legendary.

Thank you for your work.
Slawo





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