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

Robert Haas <robertmhaas@gmail.com> writes:
> In particular, as Jeff and Amit point out, it
> may well be that (a) before apply_projection_to_path(), the cheapest
> plan is non-parallel and (b) after apply_projection_to_path(), the
> cheapest plan would be a Gather plan, except that it's too late
> because we've already thrown that path out.

I'm not entirely following.  I thought that add_path was set up to treat
"can be parallelized" as an independent dimension of merit, so that
parallel paths would always survive.

> What we ought to do, I think, is avoid generating gather paths until
> after we've applied the target list (and the associated costing
> changes) to both the regular path list and the partial path list.

Might be a tad messy to rearrange things that way.
        regards, tom lane