Hi, Alexander! On Wed, May 7, 2025 at 12:06 PM Alexander Pyhalov <a.pyha...@postgrespro.ru> wrote: > Andrei Lepikhov писал(а) 2025-05-07 12:03: > > On 7/5/2025 08:57, Alexander Pyhalov wrote: > >> Andrei Lepikhov писал(а) 2025-05-07 08:02: > >>> On 5/5/2025 15:56, Alexander Pyhalov wrote: > >>>> Andrei Lepikhov писал(а) 2025-05-05 14:38: > >>>> Also logic a bit differs if path is NULL. In > >>>> get_cheapest_path_for_pathkeys_ext() we explicitly check for path > >>>> being NULL, in get_cheapest_fractional_path_for_pathkeys_ext() only > >>>> after calculating sort cost. > >>>> > >>>> I've tried to fix comments a bit and unified functions definitions. > >>> Generally seems ok, I'm not a native speaker to judge the comments. > >>> But: > >>> if (base_path && path != base_path) > >>> > >>> What is the case in your mind where the base_path pointer still may > >>> be null at that point? > >> > >> Hi. > >> > >> It seems if some childrel doesn't have valid pathlist, subpaths_valid > >> would be false in add_paths_to_append_rel() > >> and generate_orderedappend_paths() will not be called. So when we > >> iterate over live_childrels, > >> all of them will have cheapest_total path. This is why we can assert > >> that base_path is not NULL. > > I'm not sure I understand you correctly. Under which conditions will > > rel->cheapest_total_path be set to NULL for a childrel? Could you > > provide an example? > > Sorry, perhaps I was not clear enough. I've stated the opposite - it > seems we can be sure that it's not NULL.
Thank you for your work on this subject! I have the following question. I see patch changes some existing plans from Sort(Append(...)) to MergeAppend(Sort(), ..., Sort(...)) or even Materialize(MergeAppend(Sort(), ..., Sort(...))). This should be some problem in cost_sort(). Otherwise, that would mean that Sort node doesn't know how to do its job: explicit splitting dataset into pieces then merging sorting result appears to be cheaper, but Sort node contains merge-sort algorithm inside and it's supposed to be more efficient. Could you, please, revise the patch to avoid these unwanted changes? ------ Regards, Alexander Korotkov Supabase
