On Tue, Dec 4, 2012 at 11:52 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> Alexander Korotkov <aekorot...@gmail.com> writes: > > On Mon, Dec 3, 2012 at 8:31 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > >> But having said that, I'm wondering (without having read the patch) > >> why you need anything more than the existing "resjunk" field. > > > Actually, I don't know all the cases when "resjunk" flag is set. Is it > > reliable to decide target to be used only for "ORDER BY" if it's > "resjunk" > > and neither system or used in grouping? If it's so or there are some > other > > cases which are easy to determine then I'll remove "resorderbyonly" flag. > > resjunk means that the target is not supposed to be output by the query. > Since it's there at all, it's presumably referenced by ORDER BY or GROUP > BY or DISTINCT ON, but the meaning of the flag doesn't depend on that. > > What you would need to do is verify that the target is resjunk and not > used in any clause besides ORDER BY. I have not read your patch, but > I rather imagine that what you've got now is that the parser checks this > and sets the new flag for consumption far downstream. Why not just make > the same check in the planner? > > A more invasive, but possibly cleaner in the long run, approach is to > strip all resjunk targets from the query's tlist at the start of > planning and only put them back if needed. > > BTW, when I looked at this a couple years ago, it seemed like the major > problem was that the planner assumes that all plans for the query should > emit the same tlist, and thus that tlist eval cost isn't a > distinguishing factor. Breaking that assumption seemed to require > rather significant refactoring. I never found the time to try to > actually do it. > May be there is some way to not remove items from tlist, but evade actual calculation? ------ With best regards, Alexander Korotkov.