On Tue, Apr 27, 2021 at 7:45 AM Greg Nancarrow <gregn4...@gmail.com> wrote:
>
> On Tue, Apr 27, 2021 at 10:51 AM Bharath Rupireddy
> <bharath.rupireddyforpostg...@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.Agree. > > 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(). Yes, while we avoid runtime overhead, but we run the risk of changed parallel safety of any of the underlying objects/functions/partitions. This risk will anyways be unavoidable with declarative approach. > > 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. As I said upthread, we can provide the parallel safety check of associated objects/partitions as an option with default as false. I'm not sure if this is a good thing to do at all. Thoughts? With Regards, Bharath Rupireddy. EnterpriseDB: http://www.enterprisedb.com
