Re: Bloom Filter lookup for hash joins

Ants Aasma <ants(at)cybertec(dot)at> writes:
> On Wed, Jun 26, 2013 at 9:46 AM, Atri Sharma <atri(dot)jiit(at)gmail(dot)com> wrote:
>> I have been reading the current implementation of hash joins, and in
>> ExecScanHashBucket, which I understand is the actual lookup function,
>> we could potentially look at a bloom filter per bucket. Instead of
>> actually looking up each hash value for the outer relation, we could
>> just check the corresponding bloom filter for that bucket, and if we
>> get a positive, then lookup the actual values i.e. continue with our
>> current behaviour (since we could be looking at a false positive).

> The problem here is that if the hash table is in memory, doing a hash
> table lookup directly is likely to be cheaper than a bloom filter
> lookup,

Yeah. Given the plan to reduce NTUP_PER_BUCKET to 1, it's hard to see
how adding a Bloom filter phase could be anything except overhead. Even
with the current average bucket length, it doesn't sound very promising.

regards, tom lane