Re: A better way than tweaking NTUP_PER_BUCKET

On 23 June 2013 03:16, Stephen Frost <sfrost@snowman.net> wrote:

> Will think on it more.

Some other thoughts related to this...

* Why are we building a special kind of hash table? Why don't we just
use the hash table code that we in every other place in the backend.
If that code is so bad why do we use it everywhere else? That is
extensible, so we could try just using that. (Has anyone actually
tried?)

* We're not thinking about cache locality and set correspondence
either. If the join is expected to hardly ever match, then we should
be using a bitmap as a bloom filter rather than assuming that a very
large hash table is easily accessible.

* The skew hash table will be hit frequently and would show good L2
cache usage. I think I'll try adding the skew table always to see if
that improves the speed of the hash join.

-- Simon Riggs                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services