Re: Hash index todo list item

On Sun, 2007-02-09 at 13:04 -0500, Kenneth Marshall wrote:
> 2. Evaluate the performance of different hash index implementations
> and/or changes to the current implementation. My current plan is
> to keep the implementation as simple as possible and still provide
> the desired performance. Several hash index suggestions deal with
> changing the layout of the keys on a page to improve lookup
> performance, including reducing the bucket size to a fraction of
> a page or only storing the hash value on the page, instead of
> the index value itself.

You might find this patch useful:

http://archives.postgresql.org/pgsql-patches/2005-05/msg00164.php

It implements the "just store the hash in the index" idea; it also sorts
the entries in a bucket by the hash value, which allows binary search to
be used to locate candidate matches.

I was surprised that this didn't result in a performance improvement for
the benchmarks that I ran, but I never got around to investigating
further (either running more benchmarks or checking whether there was a
bug in the implementation).

Unfortunately, the patch doesn't apply cleanly to HEAD, but I can merge
it up to HEAD if you'd like.

-Neil