| From: | Peter Geoghegan <pg(at)bowt(dot)ie> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Boom filters for hash joins (was: A design for amcheck heapam verification) |
| Date: | 2017-09-18 21:13:05 |
| Message-ID: | CAH2-Wz=huVDsS8BeLp7VBtfwBHJfV8p90EWWPPe862wg4M-Hvg@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Mon, Sep 18, 2017 at 2:07 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Mon, Sep 18, 2017 at 1:29 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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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