Re: Parallel INSERT SELECT take 2

On Tue, Apr 27, 2021 at 10:51 AM Bharath Rupireddy
<bharath(dot)rupireddyforpostgres(at)gmail(dot)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.

>
> 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.

Regards,
Greg Nancarrow
Fujitsu Australia