Subject: Re: Question to datatype.sgml
To: Tatsuo Ishii <ishii@sraoss.co.jp>, pgsql-docs@lists.postgresql.org
References: <20191230.093451.1762483750956466101.t-ishii@sraoss.co.jp>
From: Vik Fearing <vik.fearing@2ndquadrant.com>
Message-ID: <79565c24-89ed-abfd-46df-4a23cde21b04@2ndquadrant.com>
Date: Mon, 30 Dec 2019 01:52:48 +0100
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On 30/12/2019 01:34, Tatsuo Ishii wrote:
> Hi,
>
> Can you please someone elaborate what's the meaning of "even" in
> datatype.sgml?
>
>     <para>
>      By default, floating point values are output in text form in their
>      shortest precise decimal representation; the decimal value produced is
>      closer to the true stored binary value than to any other value
>      representable in the same binary precision. (However, the output value is
>      currently never <emphasis>exactly</emphasis> midway between two
>      representable values, in order to avoid a widespread bug where input
>      routines do not properly respect the round-to-even rule.) This value will
>                                           ~~~~~~~~~~~~~~~~~~~
>      use at most 17 significant decimal digits for <type>float8</type>
>      values, and at most 9 digits for <type>float4</type> values.
>     </para>
>
> I think "even" means "equality" here, not "even" of "even and odd".


It does actually mean "even and odd".  This kind of rounding is also
called "Banker's Rounding".

https://en.wikipedia.org/wiki/Rounding#Round_half_to_even

-- 

Vik Fearing