Hi, Vlada. On Tue, Feb 18, 2025 at 6:56 PM Vlada Pogozhelskaya <v.pogozhelsk...@postgrespro.ru> wrote: > Following the discussion on improving statistics estimation by considering > GROUP BY as a unique constraint, I’ve prepared a patch that integrates GROUP > BY into cardinality estimation in a similar way to DISTINCT. > > This patch ensures that when a subquery contains a GROUP BY clause, the > optimizer recognizes the grouped columns as unique. The logic follows a > straightforward approach, comparing the GROUP BY columns with the target list > to determine uniqueness. > > I’d appreciate any feedback or suggestions for further improvements.
Thank you for your patch, but your message lacking of explanation on what is your approach and how is it different from previously published patches on this thread. As I get from the code, you check if group by clauses are same to targetlist. If that's true, you assume every column to be unique. But that's just doesn't work this way. If values are unique in some set of columns, individual columns might have repeats. See the example. # select x, y from generate_series(1,3) x, generate_series(1, 3) y group by x, y; x | y ---+--- 3 | 2 2 | 2 3 | 1 2 | 1 1 | 3 1 | 2 1 | 1 2 | 3 3 | 3 (9 rows) x and y are unique here as a pair. But individual x and y values have repeats. ------ Regards, Alexander Korotkov Supabase
