Hi! On Tue, Jan 21, 2014 at 3:24 AM, Marti Raudsepp <ma...@juffo.org> wrote:
> On Tue, Jan 14, 2014 at 5:49 PM, Alexander Korotkov > <aekorot...@gmail.com> wrote: > >On Tue, Jan 14, 2014 at 12:54 AM, Marti Raudsepp <ma...@juffo.org> wrote: > >> I've been trying it out in a few situations. I implemented a new > >> enable_partialsort GUC to make it easier to turn on/off, this way it's > a lot > >> easier to test. The attached patch applies on top of > partial-sort-5.patch > > > > I though about such option. Generally not because of testing convenience, > > but because of overhead of planning. This way you implement it is quite > > naive :) > > I don't understand. I had another look at this and cost_sort still > seems like the best place to implement this, since that's where the > patch decides how many pre-sorted columns to use. Both mergejoin and > simple order-by plans call into it. If enable_partialsort=false then I > skip all pre-sorted options except full sort, making cost_sort behave > pretty much like it did before the patch. > > I could change pathkeys_common to return 0, but that seems like a > generic function that shouldn't be tied to partialsort. The old code > paths called pathkeys_contained_in anyway, which has similar > complexity. (Apart for initial_cost_mergejoin, but that doesn't seem > special enough to make an exception for). > > Or should I use?: > enable_partialsort ? pathkeys_common(...) : 0 > > > For instance, merge join rely on partial sort which will be > > replaced with simple sort. > > Are you saying that enable_partialsort=off should keep > partialsort-based mergejoins enabled? > > Or are you saying that merge joins shouldn't use "simple sort" at all? > But merge join was already able to use full Sort nodes before your > patch. > Sorry that I didn't explained it. In particular I mean following: 1) With enable_partialsort = off all mergejoin logic should behave as without partial sort patch. 2) With partial sort patch get_cheapest_fractional_path_for_pathkeys function is much more expensive to execute. With enable_partialsort = off it should be as cheap as without partial sort patch. I'll try to implement this option in this week. For now, I have attempt to fix extra columns in mergejoin problem. It would be nice if you test it. ------ With best regards, Alexander Korotkov.
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