Robert Haas <robertmh...@gmail.com> writes: > On Wed, Mar 16, 2016 at 2:02 PM, Tom Lane <t...@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 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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers