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Re: BUG #1888: bug in cast from float4 to float8

From: Michael Fuhr <mike(at)fuhr(dot)org>
To: Pablo Oses <opablo(at)gmail(dot)com>
Cc: pgsql-bugs(at)postgresql(dot)org
Subject: Re: BUG #1888: bug in cast from float4 to float8
Date: 2005-09-17 13:51:36
Message-ID: 20050917135136.GA8979@winnie.fuhr.org (view raw or flat)
Thread:
Lists: pgsql-bugs
On Sat, Sep 17, 2005 at 04:00:50AM +0100, Pablo Oses wrote:
> select cast(123.456::float4 as float8)
> 
> what do you think its the answer ? 123.456 ?
> 
> well pgsql 7.2.2, 8.0.0, 8.0.1 and 8.0.3 (all i have used) answers this:
> 123.456001281738 which is WRONG

Float4 has a precision of only about 6 digits, so you can't assume
much about the digits beyond that.  See the documentation for
floating-point types:

http://www.postgresql.org/docs/8.0/interactive/datatype.html#DATATYPE-FLOAT

This is hardly a behavior unique to PostgreSQL; use a search engine
to find explanations of the problems of using binary floating-point
numbers.

If you need exact results then use the numeric type.

-- 
Michael Fuhr

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Subject: BUG #1889: little documentation error
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