Re: tweaking NTUP_PER_BUCKET

On Thu, Jul 3, 2014 at 11:40 AM, Atri Sharma <atri.jiit@gmail.com> wrote:
> IIRC, last time when we tried doing bloom filters, I was short of some real
> world useful hash functions that we could use for building the bloom filter.

Last time was we wanted to use bloom filters in hash joins to filter
out tuples that won't match any of the future hash batches to reduce
the amount of tuples that need to be spilled to disk. However the
problem was that it was unclear for a given amount of memory usage how
to pick the right size bloom filter and how to model how much it would
save versus how much it would cost in reduced hash table size.

I think it just required some good empirical tests and hash join heavy
workloads to come up with some reasonable guesses. We don't need a
perfect model just some reasonable bloom filter size that we're pretty
sure will usually help more than it hurts.


-- 
greg