Re: A reloption for partitioned tables - parallel_workers

hi Amit,

Thanks so much for doing this. I had created

https://commitfest.postgresql.org/32/2987/

and it looks like it now shows your patch as the one to use. Let me know if I should do anything else.

Best,
Seamus

On Fri, Feb 19, 2021, at 2:30 AM, Amit Langote wrote:
> On Tue, Feb 16, 2021 at 3:05 PM Amit Langote <amitlangote09(at)gmail(dot)com> wrote:
> > On Tue, Feb 16, 2021 at 1:35 AM Seamus Abshere <seamus(at)abshere(dot)net> wrote:
> > > Here we go, my first patch... solves https://www.postgresql.org/message-id/7d6fdc20-857c-4cbe-ae2e-c0ff9520ed55@www.fastmail.com
> >
> > Thanks for sending the patch here.
> >
> > It seems you haven't made enough changes for reloptions code to
> > recognize parallel_workers as valid for partitioned tables, because
> > even with the patch applied, I get this:
> >
> > create table rp (a int) partition by range (a);
> > create table rp1 partition of rp for values from (minvalue) to (0);
> > create table rp2 partition of rp for values from (0) to (maxvalue);
> > alter table rp set (parallel_workers = 1);
> > ERROR: unrecognized parameter "parallel_workers"
> >
> > You need this:
> >
> > diff --git a/src/backend/access/common/reloptions.c
> > b/src/backend/access/common/reloptions.c
> > index 029a73325e..9eb8a0c10d 100644
> > --- a/src/backend/access/common/reloptions.c
> > +++ b/src/backend/access/common/reloptions.c
> > @@ -377,7 +377,7 @@ static relopt_int intRelOpts[] =
> > {
> > "parallel_workers",
> > "Number of parallel processes that can be used per
> > executor node for this relation.",
> > - RELOPT_KIND_HEAP,
> > + RELOPT_KIND_HEAP | RELOPT_KIND_PARTITIONED,
> > ShareUpdateExclusiveLock
> > },
> > -1, 0, 1024
> >
> > which tells reloptions parsing code that parallel_workers is
> > acceptable for both heap and partitioned relations.
> >
> > Other comments on the patch:
> >
> > * This function calling style, where the first argument is not placed
> > on the same line as the function itself, is not very common in
> > Postgres:
> >
> > + /* First see if there is a root-level setting for parallel_workers */
> > + parallel_workers = compute_parallel_worker(
> > + rel,
> > + -1,
> > + -1,
> > + max_parallel_workers_per_gather
> > +
> >
> > This makes the new code look very different from the rest of the
> > codebase. Better to stick to existing styles.
> >
> > 2. It might be a good idea to use this opportunity to add a function,
> > say compute_append_parallel_workers(), for the code that does what the
> > function name says. Then the patch will simply add the new
> > compute_parallel_worker() call at the top of that function.
> >
> > 3. I think we should consider the Append parent relation's
> > parallel_workers ONLY if it is a partitioned relation, because it
> > doesn't make a lot of sense for other types of parent relations. So
> > the new code should look like this:
> >
> > if (IS_PARTITIONED_REL(rel))
> > parallel_workers = compute_parallel_worker(rel, -1, -1,
> > max_parallel_workers_per_gather);
> >
> > 4. Maybe it also doesn't make sense to consider the parent relation's
> > parallel_workers if Parallel Append is disabled
> > (enable_parallel_append = off). That's because with a simple
> > (non-parallel) Append running under Gather, all launched parallel
> > workers process the same partition before moving to the next one.
> > OTOH, one's intention of setting parallel_workers on the parent
> > partitioned table would most likely be to distribute workers across
> > partitions, which is only possible with parallel Append
> > (enable_parallel_append = on). So, the new code should look like
> > this:
> >
> > if (IS_PARTITIONED_REL(rel) && enable_parallel_append)
> > parallel_workers = compute_parallel_worker(rel, -1, -1,
> > max_parallel_workers_per_gather);
>
> Here is an updated version of the Seamus' patch that takes into
> account these and other comments received on this thread so far.
> Maybe warrants adding some tests too but I haven't.
>
> Seamus, please register this patch in the next commit-fest:
> https://commitfest.postgresql.org/32/
>
> If you haven't already, you will need to create a community account to
> use that site.
>
> --
> Amit Langote
> EDB: http://www.enterprisedb.com
>
> Attachments:
> * v2-0001-Allow-setting-parallel_workers-on-partitioned-tab.patch