Re: PATCH: hashjoin - gracefully increasing NTUP_PER_BUCKET instead of batching

On Thu, Dec 11, 2014 at 12:29 PM, Kevin Grittner <kgrittn@ymail.com> wrote:
> Robert Haas <robertmhaas@gmail.com> wrote:
>> On Sat, Dec 6, 2014 at 10:08 PM, Tomas Vondra <tv@fuzzy.cz> wrote:
>>>    select a.i, b.i from a join b on (a.i = b.i);
>>
>> I think the concern is that the inner side might be something more
>> elaborate than a plain table scan, like an aggregate or join.  I might
>> be all wet, but my impression is that you can make rescanning
>> arbitrarily expensive if you work at it.
>
> I'm not sure I'm following.  Let's use a function to select from b:
>
> create or replace function fb()
>   returns setof b
>   language plpgsql
>   rows 1
> as $$
> begin
>   return query select i from b;
> end;
> $$;
>
> explain (analyze, buffers, verbose)
>   select a.i, b.i from a join fb() b on (a.i = b.i);
>
> I used the low row estimate to cause the planner to put this on the inner side.
>
> 16 batches
> Execution time: 1638.582 ms
>
> Now let's make it slow.
>
> create or replace function fb()
>   returns setof b
>   language plpgsql
>   rows 1
> as $$
> begin
>   perform pg_sleep(2.0);
>   return query select i from b;
> end;
> $$;
> explain (analyze, buffers, verbose)
>   select a.i, b.i from a join fb() b on (a.i = b.i);
>
> 16 batches
> Execution time: 3633.859 ms
>
> Under what conditions do you see the inner side get loaded into the
> hash table multiple times?

Huh, interesting.  I guess I was thinking that the inner side got
rescanned for each new batch, but I guess that's not what happens.

Maybe there's no real problem here, and we just win.

-- 
Robert Haas
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