Re: A better way than tweaking NTUP_PER_BUCKET

On 22 June 2013 21:40, Stephen Frost <sfrost@snowman.net> wrote:

> I'm actually not a huge fan of this as it's certainly not cheap to do. If it
> can be shown to be better than an improved heuristic then perhaps it would
> work but I'm not convinced.

We need two heuristics, it would seem:

* an initial heuristic to overestimate the number of buckets when we
have sufficient memory to do so

* a heuristic to determine whether it is cheaper to rebuild a dense
hash table into a better one.

Although I like Heikki's rebuild approach we can't do this every x2
overstretch. Given large underestimates exist we'll end up rehashing
5-12 times, which seems bad. Better to let the hash table build and
then re-hash once, it we can see it will be useful.

OK?

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