Robert Haas <robertmhaas@gmail.com> writes:
> On Wed, Mar 16, 2016 at 2:02 PM, Tom Lane <tgl@sss.pgh.pa.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
toavoid modifying the subpath in place. * It seems unlikely at present that there could be any other *
referencesto 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