Re: [PERFORM] "Hash index" vs. "b-tree index" (PostgreSQL

Greg Stark <gsstark(at)mit(dot)edu> writes:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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