Re: Parallel INSERT SELECT take 2 - Mailing list pgsql-hackers

On Tue, Apr 27, 2021 at 7:45 AM Greg Nancarrow <gregn4422@gmail.com> wrote:
>
> On Tue, Apr 27, 2021 at 10:51 AM Bharath Rupireddy
> <bharath.rupireddyforpostgres@gmail.com> wrote:
> >
> >
> > I still feel that why we shouldn't limit the declarative approach to
> > only partitioned tables? And for normal tables, possibly with a
> > minimal cost(??), the server can do the safety checking. I know this
> > feels a little inconsistent. In the planner we will have different
> > paths like: if (partitioned_table) { check the parallel safety tag
> > associated with the table } else { perform the parallel safety of the
> > associated objects }.
> >
>
> Personally I think the simplest and best approach is just do it
> consistently, using the declarative approach across all table types.
>

Yeah, if we decide to go with a declarative approach then that sounds
like a better approach.

> >
> > Then, running the pg_get_parallel_safety will have some overhead if
> > there are many partitions associated with a table. And, this is the
> > overhead planner would have had to incur without the declarative
> > approach which we are trying to avoid with this design.
> >
>
> The big difference is that pg_get_parallel_safety() is intended to be
> used during development, not as part of runtime parallel-safety checks
> (which are avoided using the declarative approach).
> So there is no runtime overhead associated with pg_get_parallel_safety().
>
> >
> > I'm thinking that when users say ALTER TABLE partioned_table SET
> > PARALLEL TO 'safe';, we check all the partitions' and their associated
> > objects' parallel safety? If all are parallel safe, then only we set
> > partitioned_table as parallel safe. What should happen if the parallel
> > safety of any of the associated objects/partitions changes after
> > setting the partitioned_table safety?
> >
>
> With the declarative approach, there is no parallel-safety checking on
> either the CREATE/ALTER when the parallel-safety is declared/set.
> It's up to the user to get it right. If it's actually wrong, it will
> be detected at runtime.
>

OTOH, even if we want to verify at DDL time, we won't be able to
maintain it at the later point of time say if user changed the
parallel-safety of some function used in check constraint. I think the
function pg_get_parallel_safety() will help the user to decide whether
it can declare table parallel-safe. Now, it is quite possible that the
user can later change the parallel-safe property of some function then
that should be caught at runtime.

-- 
With Regards,
Amit Kapila.