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

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Wed, Mar 16, 2016 at 2:02 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Looks pretty close. One point is that if we do end up using a Result
>> node, then the parent GatherPath does not get charged for the Result
>> node's cpu_per_tuple overhead. I'm not sure that that's worth changing
>> though. It's probably better to bet that the subpath is projectable and
>> so no cost will ensue, than to bet the other way.

> I'm almost sure this way is the better bet.

Actually, we do know what will happen ... so maybe

/*
* We always use create_projection_path here, even if the subpath is
* projection-capable, so as to avoid modifying the subpath in place.
* It seems unlikely at present that there could be any other
* references to the subpath anyway, but better safe than sorry.
*/
+ if (!is_projection_capable_path(gpath->subpath))
+ gpath->path.total_cost += cpu_tuple_cost * gpath->subpath->rows;
gpath->subpath = (Path *)
create_projection_path(root,
gpath->subpath->parent,
gpath->subpath,
target);

The comment could use adjustment if you adopt that, to reference the fact
that we know create_projection_plan will get rid of the Result if not
needed.

regards, tom lane