I traced that to the fact that reparameterize_path_by_child() omits support for MaterialPath, so that if the only surviving path(s) for a child join include materialization steps, we'll fail outright to produce a plan for the parent join.
Yeah, that's true. It's weird we neglect MaterialPath here.
Unfortunately, I don't have an example that produces such a failure against HEAD. It seems certain to me that such cases exist, though, so I'd like to apply and back-patch the attached.
I tried on HEAD and got one, which leverages sampled rel to generate the MaterialPath and lateral reference to make it the only available path.
SET enable_partitionwise_join to true;
CREATE TABLE prt (a int, b int) PARTITION BY RANGE(a); CREATE TABLE prt_p1 PARTITION OF prt FOR VALUES FROM (0) TO (10);
CREATE EXTENSION tsm_system_time;
explain (costs off) select * from prt t1 left join lateral (select t1.a as t1a, t2.a as t2a from prt t2 TABLESAMPLE system_time (10)) ss on ss.t1a = ss.t2a; ERROR: could not devise a query plan for the given query
