Re: make Gather node projection-capable

On Thu, Oct 22, 2015 at 2:49 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> You probably would, but sometimes that might not be possible; for
> example, the tlist might contain a parallel-restricted function (which
> therefore has to run in the leader).

Oh, also: pushing down the tlist is actually sorta non-trivial at the
moment.  I can stick a GatherPath on top of whatever the join planner
kicks out (although I don't have a cost model for this yet, so right
now it's just a hard-coded test), but the upper planner substitutes
the tlist into whatever the topmost plan node is - and that's the
Gather, not whatever's under it.  Maybe I should have a special case
for this: if the node into which we would insert the final tlist is a
Gather, see whether it's parallel-safe, and if so, peel the Gather
node off, apply the tlist to whatever's left (adding a gating Result
node if need be) and put the Gather back on.  This seems less
important than a few other things I need to get done, but certainly
worth doing.  But do you know whether the upper planner path-ifaction
work will be likely to render whatever code I might write here
obsolete?

-- 
Robert Haas
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