Re: Re: [GENERAL] +/- Inf for float8's - Mailing list pgsql-hackers

My assumption is that we never came up with any solution to this, right?


> On Sun, Aug 20, 2000 at 12:33:00AM +0200, Peter Eisentraut wrote:
> <snip side comment about bug tracking. My input: for an email controllable
> system, take a look at the debian bug tracking system>
> 
> > Show me a system where it doesn't work and we'll get it to work.
> > UNSAFE_FLOATS as it stands it probably not the most appropriate behaviour;
> > it intends to speed things up, not make things portable.
> > 
> 
> I agree. In the previous thread on this, Thomas suggested creating a flag
> that would allow control turning the  CheckFloat8Val function calls into
> a macro NOOP. Sound slike a plan to me.
> 
> > 
> > > > NULL and NaN are not quite the same thing imho. If we are allowing NaN
> > > > in columns, then it is *known* to be NaN.
> > > 
> > > For the purposes of ordering, however, they are very similar.
> > 
> > Then we can also treat them similar, i.e. sort them all last or all first.
> > If you have NaN's in your data you wouldn't be interested in ordering
> > anyway.
> 
> Right, but the problem is that NULLs are an SQL language feature, and
> there for rightly special cased directly in the sorting apparatus. NaN is
> type specific, and I'd be loath to special case it in the same place. As
> it happens, I've spent some time this weekend groveling through the sort
> (and index, as it happens) code, and have an idea for a type specific fix.
> 
> Here's the deal, and an actual, honest to goodness bug in the current code.
> 
> As it stands, we allow one non-finite to be stored in a float8 field:
> NaN, with partial parsing of 'Infinity'.
> 
> As I reported last week, NaNs break sorts: they act as barriers, creating
> sorted subsections in the output.  As those familiar with the code have
> already guessed, there is a more serious bug: NaNs break indicies on
> float8 fields, essentially chopping the index off at the first NaN.
> 
> Fixing this turns out to be a one liner to btfloat8cmp.
> 
> Fixing sorts is a bit tricker, but can be done: Currently, I've hacked
> the float8lt and float8gt code to sort NaN to after +/-Infinity. (since
> NULLs are special cased, they end up sorting after NaN). I don't see
> any problems with this solution, and it give the desired behavior.
> 
> I've attached a patch which fixes all the sort and index problems, as well
> as adding input support for -Infinity. This is not a complete solution,
> since I haven't done anything with the CheckFloat8Val test. On my
> system (linux/glibc2.1) compiling with UNSAFE_FLOATS seems to work fine 
> for testing.
> 
> > 
> > Side note 2: The paper "How Java's floating point hurts everyone
> > everywhere" provides for good context reading.
> 
> http://http/cs.berkeley.edu/~wkahan/JAVAhurt.pdf ? I'll take a look at it
> when I get in to work Monday.
> 
> > 
> > Side note 3: Once you read that paper you will agree that using floating
> > point with Postgres is completely insane as long as the FE/BE protocol is
> > text-based.
> 
> Probably. But it's not our job to enforce sanity, right? Another way to think
> about it is fixing the implementation so the deficiencies of the FE/BE stand
> out in a clearer light. ;-)
> 
> Ross
> -- 
> Ross J. Reedstrom, Ph.D., <reedstrm@rice.edu> 
> NSBRI Research Scientist/Programmer
> Computer and Information Technology Institute
> Rice University, 6100 S. Main St.,  Houston, TX 77005
> 

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