Re: Parallel INSERT (INTO ... SELECT ...)

On Wed, Mar 10, 2021 at 8:18 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> Now, coming back to Hou-San's patch to introduce a GUC and reloption
> for this feature, I think both of those make sense to me because when
> the feature is enabled via GUC, one might want to disable it for
> partitioned tables? Do we agree on that part or someone thinks
> otherwise?
>

Having both makes sense to me.

> The other points to bikeshed could be:
> 1. The name of GUC and reloption. The two proposals at hand are
> enable_parallel_dml and enable_parallel_insert. I would prefer the
> second (enable_parallel_insert) because updates/deletes might not have
> a similar overhead.
>

I guess to have the finer granularity we'd have to go with
enable_parallel_insert, which then would mean possibly having to later
add enable_parallel_update, should parallel update have similar
potential overhead in the parallel-safety checks (which to me, looks
like it could, and parallel delete may not ...)

It's a shame there is no "set" type for GUC options.
e.g.
enable_parallel_dml='insert,update'
Maybe that's going too far.

> 2. Should we keep the default value of GUC to on or off? It is
> currently off. I am fine keeping it off for this release and we can
> always turn it on in the later releases if required. Having said that,
> I see the value in keeping it on because in many cases Insert ...
> Select will be used for large data and there we will see a benefit of
> parallelism and users facing trouble (who have a very large number of
> partitions with less data to query) can still disable the parallel
> insert for that particular table. Also, the other benefit of keeping
> it on till at least the beta period is that this functionality will
> get tested and if we found reports of regression then we can turn it
> off for this release as well.
>

I'd agree to keeping it on by default (and it can be selectively
turned off for a particular table using the reloption, if needed).

Regards,
Greg Nancarrow
Fujitsu Australia