
Regarding inter-data-type hash joins, would it make sense to make float
datatypes to hash to the same value as integral data types for integral
values?

Conveniently this would cover the existing special case of -0 and +0 hashing
to the same value. Something like this?

Datum
hashfloat4(PG_FUNCTION_ARGS)
{
	float4		key = PG_GETARG_FLOAT4(0);

        if (key == (int32)key)
		PG_RETURN_UINT32(~(int32)key);

	return hash_any((unsigned char *) &key, sizeof(key));
}


Incidentally, why do all the floatfoo functions invert the value? It doesn't
seem like that affects the quality of the hash at all.


-- 
greg


04 May 2004 18:45:37 -0400
Greg Stark <gsstark@mit.edu>
More Hashing questions
If you do this I suggest supporting bigint as well.

On Tue, May 04, 2004 at 06:45:37PM -0400, Greg Stark wrote:
> 
> Regarding inter-data-type hash joins, would it make sense to make float
> datatypes to hash to the same value as integral data types for integral
> values?
> 
> Conveniently this would cover the existing special case of -0 and +0 hashing
> to the same value. Something like this?
> 
> Datum
> hashfloat4(PG_FUNCTION_ARGS)
> {
> 	float4		key = PG_GETARG_FLOAT4(0);
> 
>         if (key == (int32)key)
> 		PG_RETURN_UINT32(~(int32)key);
> 
> 	return hash_any((unsigned char *) &key, sizeof(key));
> }
> 
> 
> Incidentally, why do all the floatfoo functions invert the value? It doesn't
> seem like that affects the quality of the hash at all.
> 
> 
> -- 
> greg
> 
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Don't 'kill -9' the postmaster
> 

-- 
Jim C. Nasby, Database Consultant                  jim@nasby.net
Member: Triangle Fraternity, Sports Car Club of America
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Thu, 6 May 2004 09:38:30 -0500
"Jim C. Nasby" <jim@nasby.net>
Re: More Hashing questions

"Jim C. Nasby" <jim@nasby.net> writes:

> If you do this I suggest supporting bigint as well.

Well I'm not about to actually do the inter-data-type hash joins. I was just
asking if the hash functions for floats should be adjusted to guarantee this
property. bigints already guarantee it, though I think the hash function can
be improved without breaking it.

> On Tue, May 04, 2004 at 06:45:37PM -0400, Greg Stark wrote:
> > 
> > Regarding inter-data-type hash joins, would it make sense to make float
> > datatypes to hash to the same value as integral data types for integral
> > values?

I have a little patch but I have to test it, at least try to compile it,
before I send it anywhere :)

-- 
greg


06 May 2004 16:33:34 -0400
Greg Stark <gsstark@mit.edu>
Re: More Hashing questions