On Wed, Mar 16, 2016 at 2:25 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > 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.
OK, I've committed something along those lines. Thanks for the advice, and feel free to whack it around if you have an idea for improving it further - though IMHO this is good enough for 9.6. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers