Re: Horribly slow hash join

Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

> Greg Stark <gsstark(at)mit(dot)edu> writes:
> > Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
> >> (Also, I have been harboring some notions of supporting cross-type hash
> >> joins for integer types, which will not work unless small int8 values hash
> >> the same as int4 etc.)
>
> > The obvious way to modify the hash function is to xor the high 32 bits with
> > the low 32 bits. That maintains the property you need
>
> No it doesn't ...

Eh? Oh, negative numbers? So low^high^sign.

I wonder if it makes sense to have check the hash distribution after
generating the table and if it's bad then throw it away and try again with a
different hash function. The "different hash function" would probably just be
a seed value changing. Probably way overkill though.

--
greg