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

Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Sat, Dec 6, 2014 at 10:08 PM, Tomas Vondra <tv(at)fuzzy(dot)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?

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company