Re: [HACKERS] why not parallel seq scan for slow functions

On Sun, Jan 28, 2018 at 10:13 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> If we want to get rid of Gather (Merge) checks in
> apply_projection_to_path(), then we need some way to add a projection
> path to the subpath of gather node even if that is capable of
> projection as we do now.  I think changing the order of applying
> scan/join target won't address that unless we decide to do it for
> every partial path.  Another way could be that we handle that in
> generate_gather_paths, but I think that won't be the idle place to add
> projection.
>
> If we want, we can compute the parallel-safety of scan/join target
> once in grouping_planner and then pass it in apply_projection_to_path
> to address your main concern.

I spent some time today hacking on this; see attached.  It needs more
work, but you can see what I have in mind.  It's not quite the same as
what I outlined before because that turned out to not quite work, but
it does remove most of the logic from apply_projection_to_path().

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company