On Thu, Jul 14, 2022 at 10:02 PM Antonin Houska <ah@cybertec.at> wrote:
I'd prefer a test that demonstrates that the Gather node at the top of the "subproblem plan" is useful purely from the *cost* perspective, rather than due to executor limitation.
This patch provides an additional path (Gather atop of subproblem) which was not available before. But your concern makes sense that we need to show this new path is valuable from competing on cost with other paths.
How about we change to Nested Loop at the topmost? Something like:
Maybe a better example is that we use a small table 'c' to avoid the Gather node above scanning 'c', so that the path of parallel nestloop is possible to be generated.
set join_collapse_limit to 2;
# explain (costs off) select * from a join b on a.i = b.i join c on b.i > c.i; QUERY PLAN ------------------------------------------------ Nested Loop Join Filter: (b.i > c.i) -> Seq Scan on c -> Gather Workers Planned: 4 -> Parallel Hash Join Hash Cond: (a.i = b.i) -> Parallel Seq Scan on a -> Parallel Hash -> Parallel Seq Scan on b (10 rows)