Re: Hash index todo list item

Ühel kenal päeval, N, 2007-09-06 kell 09:38, kirjutas Tom Lane:
> Hannu Krosing <hannu(at)skype(dot)net> writes:
> > Ühel kenal päeval, E, 2007-09-03 kell 19:55, kirjutas Tom Lane:
> >> No, because part of the deal is that you can do lookups using only the
> >> leading index columns. At least, all the existing multicolumn index
> >> types can do that.
>
> > One approahc is not to mix hashes, but to partition the hash, so that
> > each column gets its N bits in the hash.
>
> How does that help? You still need all the keys to find out which
> bucket to look in.

no. you need to look at only the buckets where that part of hash matches

say you allocate bits 4-7 for column 2 and then need to look up column 2
value with hash 3 . here you need to look at only buckets N*16 + 3, that
is, you need to examine only each 16th bucket

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Hannu