Pushdown target list below gather node (WAS Re: WIP: Upper planner pathification)

On Wed, Mar 9, 2016 at 11:58 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Wed, Mar 9, 2016 at 12:33 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >
> > Gather is a bit weird, because although it can project (and needs to,
> > per the example of needing to compute a non-parallel-safe function),
> > you would rather push down as much work as possible to the child node;
> > and doing so is semantically OK for parallel-safe functions.  (Pushing
> > functions down past a Sort node, for a counterexample, is not so OK
> > if you are concerned about function evaluation order, or even number
> > of executions.)
> >
> > In the current code structure it would perhaps be reasonable to teach
> > apply_projection_to_path about that --- although this would require
> > logic to separate parallel-safe and non-parallel-safe subexpressions,
> > which doesn't quite seem like something apply_projection_to_path
> > should be doing.
>
> I think for v1 it would be fine to make this all-or-nothing; that's
> what I had in mind to do.  That is, if the entire tlist is
> parallel-safe, push it all down.  If not, let the workers just return
> the necessary Vars and have Gather compute the final tlist.
>