Re: Parallel Inserts (WAS: [bug?] Missed parallel safety checks..)

On Mon, Aug 2, 2021 at 2:52 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Jul 30, 2021 at 6:53 PM houzj(dot)fnst(at)fujitsu(dot)com
> <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> >
> > On Friday, July 30, 2021 2:52 PM Greg Nancarrow <gregn4422(at)gmail(dot)com> wrote:
> > > On Fri, Jul 30, 2021 at 4:02 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > >
> > > > > Besides, I think we need a new default value about parallel dml
> > > > > safety. Maybe 'auto' or 'null'(different from
> > > > > safe/restricted/unsafe). Because, user is likely to alter the safety
> > > > > to the default value to get the automatic safety check, a independent default
> > > > > value can make it more clear.
> > > > >
> > > >
> > > > Hmm, but auto won't work for partitioned tables, right? If so, that
> > > > might appear like an inconsistency to the user and we need to document
> > > > the same. Let me summarize the discussion so far in this thread so
> > > > that it is helpful to others.
> > > >
> > >
> > > To avoid that inconsistency, UNSAFE could be the default for partitioned tables
> > > (and we would disallow setting AUTO for these).
> > > So then AUTO is the default for non-partitioned tables only.
> >
> > I think this approach is reasonable, +1.
> >
>
> I see the need to change to default via Alter Table but I am not sure
> if Auto is the most appropriate way to handle that. How about using
> DEFAULT itself as we do in the case of REPLICA IDENTITY? So, if users
> have to alter parallel safety value to default, they need to just say
> Parallel DML DEFAULT. The default would mean automatic behavior for
> non-partitioned relations and ignore parallelism for partitioned
> tables.
>

Hmm, I'm not so sure I'm sold on that.
I personally think "DEFAULT" here is vague, and users then need to
know what DEFAULT equates to, based on the type of table (partitioned
or non-partitioned table).
Also, then there's two ways to set the actual "default" DML
parallel-safety for partitioned tables: DEFAULT or UNSAFE.
At least "AUTO" is a meaningful default option name for
non-partitioned tables - "automatic" parallel-safety checking, and the
fact that it isn't the default (and can't be set) for partitioned
tables highlights the difference in the way being proposed to treat
them (i.e. use automatic checking only for non-partitioned tables).
I'd be interested to hear what others think.
I think a viable alternative would be to record whether an explicit
DML parallel-safety has been specified, and if not, apply default
behavior (i.e. by default use automatic checking for non-partitioned
tables and treat partitioned tables as UNSAFE). I'm just not sure
whether this kind of distinction (explicit vs implicit default) has
been used before in Postgres options.

Regards,
Greg Nancarrow
Fujitsu Australia