Re: [HACKERS] Boom filters for hash joins (was: A design for amcheckheapam verification)

On Mon, Sep 18, 2017 at 2:07 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Mon, Sep 18, 2017 at 1:29 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Uh, why does the planner need to be involved at all?
>
> Because it loses if the Bloom filter fails to filter anything.  That's
> not at all far-fetched; consider SELECT * FROM a.x, b.x WHERE a.x =
> b.x given a foreign key on a.x referencing b(x).

Wouldn't a merge join be a lot more likely in this case anyway? Low
selectivity hash joins with multiple batches are inherently slow; the
wasted overhead of using a bloom filter may not matter.

Obviously this is all pretty speculative. I suspect that this could be
true, and it seems worth investigating that framing of the problem
first.

-- 
Peter Geoghegan


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