On Thu, 14 Sept 2023 at 02:28, Damir Belyalov <dam.be...@gmail.com> wrote: > create table a (n int); > insert into a (n) select x from generate_series(1, 140000) as g(x); > create unique index on a (n); > explain select distinct n from a; > QUERY PLAN > ------------------------------------------------------------------------------------ > Unique (cost=0.42..6478.42 rows=140000 width=4) > -> Index Only Scan using a_n_idx on a (cost=0.42..6128.42 rows=140000 > width=4) > (2 rows) > > > We can see that Unique node is redundant for this case. So I implemented a > simple patch that removes Unique node from the plan.
I don't think this is a good way to do this. The method you're using only supports this optimisation when querying a table directly. If there were subqueries, joins, etc then it wouldn't work as there are no unique indexes. You should probably have a look at [1] to see further details of an alternative method without the said limitations. David [1] https://postgr.es/m/flat/CAKU4AWqZvSyxroHkbpiHSCEAY2C41dG7VWs%3Dc188KKznSK_2Zg%40mail.gmail.com
