Re: parallel vacuum options/syntax

Le ven. 3 janv. 2020 à 09:06, Amit Kapila <[email protected]> a
écrit :

> On Thu, Jan 2, 2020 at 7:09 PM Guillaume Lelarge <[email protected]>
> wrote:
> >
> > Le jeu. 2 janv. 2020 à 13:09, Amit Kapila <[email protected]> a
> écrit :
> >>
> >> If parallel vacuum is *not* enabled by default, then I think the
> current way to enable is fine which is as follows:
> >> Vacuum (Parallel 2) <tbl_name>;
> >>
> >> Here, if the user doesn't specify parallel_degree, then we internally
> decide based on number of indexes that support a parallel vacuum with a
> maximum of max_parallel_maintenance_workers.
> >>
> >> If the parallel vacuum is enabled by default, then I could think of the
> following ways:
> >> (a) Vacuum (disable_parallel) <tbl_name>;  Vacuum (Parallel
> <parallel_degree>) <tbl_name>;
> >> (b) Vacuum (Parallel <parallel_degree>) <tbl_name>;  If user specifies
> parallel_degree as 0, then disable parallelism.
> >> (c) ... Any better ideas?
> >>
> >
> > AFAICT, every parallel-able statement use parallelisation by default, so
> it wouldn't be consistent if VACUUM behaves some other way.
> >
>
> Fair enough.
>
> > So, (c) has my vote.
> >
>
> I don't understand this.  What do you mean by voting (c) option?  Do
> you mean that you didn't like any of (a) or (b)?


I meant (b), sorry :)

  If so, then feel
> free to suggest something else.  One more possibility could be to
> allow users to specify parallel degree or disable parallelism via guc
> 'max_parallel_maintenance_workers'.  Basically, if the user wants to
> disable parallelism, it needs to set the value of guc
> max_parallel_maintenance_workers as zero and if it wants to increase
> the parallel degree than the default value (which is two), then it can
> set it via max_parallel_maintenance_workers before running vacuum
> command.  Now, this can certainly work, but I feel setting/resetting a
> guc before a vacuum
> command can be a bit inconvenient for users, but if others prefer that
> way, then we can do that.
>
>

-- 
Guillaume.