Re: pgsql: Add parallel-aware hash joins.

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2018-01-24 17:18:26 -0300, Alvaro Herrera wrote:
>> Yeah, I proposed this a decade ago but never had the wits to write the
>> code.

> It shouldn't be too hard, right? Leaving defining the file format,
> parsing it, creating the new schedule with depencencies and adapting
> tests aside (hah), it mostly seems a relatively simple graph ordering /
> topological sort problem, right?

Yeah. We already have topo sort code in pg_dump, maybe we could push that
into someplace like src/common or src/fe_utils? Although pg_dump hasn't
got any need for edge weights, so maybe sharing code isn't worth it.

We could flush the existing schedule files and use a simple format like
testname: list of earlier tests it depends on
(I guess there would be more properties than just the dependencies,
but still not hard to parse.)

> If we keep the timings from an earlier
> run somwhere, we can use the timing of runs as edge weights, making the
> schedule better.

I think we could just use constant values hand-coded in the schedule file.
It might occasionally be worth updating them, but realistically it's not
going to matter that they be very accurate. Probably weights like 1, 2,
and 3 would be plenty ;-)

regards, tom lane