Re: Parallel query behaving different with custom GUCs

On Thu, Oct 26, 2023 at 3:10 AM Rushabh Lathia <rushabh(dot)lathia(at)gmail(dot)com> wrote:
> -- RESET the custom GUC and rerun the query.
> postgres=> reset myapp.blah;
> RESET
>
> -- Query should still run, but with forcing parallel plan, throwing an error.
> postgres=> select count(*) from ptest2 where current_setting('myapp.blah') is null;
> ERROR: unrecognized configuration parameter "myapp.blah"
> CONTEXT: parallel worker
>
> -- Disable the parallel plan and query just runs fine.
> postgres=#set max_parallel_workers_per_gather to 0;
> SET
> postgres=#select count(*) from ptest2 where current_setting('myapp.blah') is null;
> count
> -------
> 0
> (1 row)
>
> We might need another placeholder for the custom GUCs. Currently, we are
> maintaining 3 linked lists in guc.c - guc_nondef_list, guc_stack_list,
> guc_report_list and to fix the above issue either we need a 4th list or do
> changes in the existing list.

I discussed this a bit with Rushabh off-list before he posted, and was
hoping someone else would reply, but since no one has:

Formally, I think this is a bug. However, the practical impact of it
is fairly low, because you have to be using custom GUCs in your query
and you have to RESET them instead of using SET to put them back to
the default value, which I'm guessing is something that not a lot of
people do. I'm a bit concerned that adding the necessary tracking
could be expensive, and I'm not sure we want to slow down things in
normal cases to cater to this somewhat strange case. On the other
hand, maybe we can fix it without too much cost, in which case that
would be good to do.

I'm also alert to my own possible bias. Perhaps since I designed this
mechanism, I'm more prone to viewing its deficiencies as minor than a
neutral observer would be. So if anyone is sitting there reading this
and thinking "wow, I can't believe Robert doesn't think it's important
to fix this," feel free to write back and say so.

--
Robert Haas
EDB: http://www.enterprisedb.com