On Wed, 24 May 2023 at 15:54, Bruce Momjian <bruce@momjian.us> wrote:
>
> On Wed, May 24, 2023 at 08:37:45AM +1200, David Rowley wrote:
> > On Mon, 22 May 2023 at 07:05, Jonathan S. Katz <jkatz@postgresql.org> wrote:
> > > * Parallel execution of queries that use `FULL` and `OUTER` joins
> >
> > I think this should be `RIGHT` joins rather than `OUTER` joins.
> >
> > LEFT joins have been parallelizable I think for a long time now.
>
> Well, since we can swap left/right easily, why would we not have just
> have swappted the tables and done the join in the past? I think there
> are two things missing in my description.
>
> First, I need to mention parallel _hash_ join. Second, I think this
> item is saying that the _inner_ side of a parallel hash join can be an
> OUTER or FULL join. How about?
>
> Allow hash joins to be parallelized where the inner side is
> processed as an OUTER or FULL join (Melanie Plageman, Thomas Munro)
>
> In this case, the inner side is the hashed side.
I think Jonathan's text is safe to swap OUTER to RIGHT as it mentions
"execution". For the release notes, maybe the mention of it can be
moved away from "E.1.3.1.1. Optimizer" and put under "E.1.3.1.2.
General Performance" and ensure we mention that we're talking about
the executor?
I'm thinking it might be confusing if we claim that this is something
that we switched on in the planner. It was a limitation with the
executor which the planner was just onboard with not producing plans
for.
David