Greg Stark <gsstark@mit.edu> writes:
> Tom Lane <tgl@sss.pgh.pa.us> writes:
>> I think that efficient implementation of this would require explicitly
>> storing the hash code for each index entry,
> It seems that means doubling the size of the hash index. That's a pretty big
> i/o to cpu tradeoff.
Hardly. The minimum possible size of a hash entry today is 8 bytes
header plus 4 bytes datum, plus there's a 4-byte line pointer to factor
in. So under the most pessimistic assumptions, storing the hash code
would add 25% to the size. (On MAXALIGN=8 hardware, it might cost you
nothing at all.)
> What if the hash index stored *only* the hash code? That could be useful for
> indexing large datatypes that would otherwise create large indexes.
Hmm, that could be a thought.
regards, tom lane