Re: Problem about postponing gathering partial paths for topmost scan/join rel
| От | Richard Guo |
|---|---|
| Тема | Re: Problem about postponing gathering partial paths for topmost scan/join rel |
| Дата | |
| Msg-id | CAMbWs49bnXzik4ugysngSY+rZ5xVtXUOh9tSgYixu3z0VUJzrQ@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Problem about postponing gathering partial paths for topmost scan/join rel (Richard Guo <guofenglinux@gmail.com>) |
| Ответы |
Re: Problem about postponing gathering partial paths for topmost scan/join rel
|
| Список | pgsql-hackers |
On Fri, Jul 15, 2022 at 4:03 PM Richard Guo <guofenglinux@gmail.com> wrote:
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)
Thanks
Richard
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)
Thanks
Richard
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