út 28. 5. 2024 v 9:48 odesílatel David Rowley <dgrowle...@gmail.com> napsal:
> On Tue, 28 May 2024 at 19:31, Pavel Stehule <pavel.steh...@gmail.com> > wrote: > > My question is - does memoize support subqueries? And can be enhanced to > support this exercise without LATERAL and optimization fences? > > It's only currently considered for parameterized nested loop joins, > not for subplans. > > I wrote a bit about this in [1] and there's even a patch. The problem > with it is that we plan subqueries and generate an actual plan before > planning the outer query. This means we don't have an ndistinct > estimate for the parameters to the subquery when we plan it, therefore > we can't tell if Memoize is a good choice or not. It isn't a good > choice if each set of parameters the subplan is called with is unique. > That would always be a cache miss and would only result in making the > query run more slowly. > > I imagined making this work by delaying the plan creation for > subqueries until the same time as create_plan() for the outer query. > If we have a Path with and without a Memoize node, at some point after > planning the outer query, we can choose which Path is the cheapest > based on the ndistinct estimate for the parameters. > Thank you for explanation Pavel > > David > > [1] > https://www.postgresql.org/message-id/CAApHDvpGX7RN%2Bsh7Hn9HWZQKp53SjKaL%3DGtDzYheHWiEd-8moQ%40mail.gmail.com >
