Hannu Krosing wrote:
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
I don't like the truncating hash suggestion because it limits the
ability of a hash code to uniquely identify a key.
If a user requires the ability to search on both (column1) and (column1,
column2), they can create two hash indexes and the planner can decide
which to use.
Or, they can use a btree. I think hash has a subset of uses where it
would be a significant gain, and focus should be spent on this subset.
Cheers,
mark
--
Mark Mielke <[EMAIL PROTECTED]>
