Re: [HACKERS] Block level parallel vacuum

On Tue, Oct 15, 2019 at 3:55 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Tue, Oct 15, 2019 at 10:34 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Mon, Oct 14, 2019 at 6:37 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> >
> > > 3. Do we really need to give the responsibility of deleting empty
> > > pages (gistvacuum_delete_empty_pages) to gistvacuumcleanup. Can't we
> > > do it in gistbulkdelte? I see one advantage of postponing it till the
> > > cleanup phase which is if somehow we can accumulate stats over
> > > multiple calls of gistbulkdelete, but I am not sure if it is feasible.
> > > At least, the way current code works, it seems that there is no
> > > advantage to postpone deleting empty pages till the cleanup phase.
> > >
> >
> > Considering the current strategy of page deletion of gist index the
> > advantage of postponing the page deletion till the cleanup phase is
> > that we can do the bulk deletion in cleanup phase which is called at
> > most once. But I wonder if we can do the page deletion in the similar
> > way to btree index.
> >
>
> I think there might be some advantage of the current strategy due to
> which it has been chosen. I was going through the development thread
> and noticed some old email which points something related to this.
> See [1].

Thanks.

>
> > Or even we use the current strategy I think we can
> > do that while not passing the pages information from bulkdelete to
> > vacuumcleanup using by GistBulkDeleteResult.
> >
>
> Yeah, I also think so. I have started a new thread [2] to know the
> opinion of others on this matter.
>

Thank you.

> > > If we avoid postponing deleting empty pages till the cleanup phase,
> > > then we don't have the problem for gist indexes.
> >
> > Yes. But considering your pointing out I guess that there might be
> > other index AMs use the stats returned from bulkdelete in the similar
> > way to gist index (i.e. using more larger structure of which
> > IndexBulkDeleteResult is just the first field). If we have the same
> > concern the parallel vacuum still needs to deal with that as you
> > mentioned.
> >
>
> Right, apart from some functions for memory allocation/estimation and
> stats copy, we might need something like amcanparallelvacuum, so that
> index methods can have the option to not participate in parallel
> vacuum due to reasons similar to gist or something else. I think we
> can work towards this direction as this anyway seems to be required
> and till we reach any conclusion for gist indexes, you can mark
> amcanparallelvacuum for gist indexes as false.

Agreed. I'll create a separate patch to add this callback and change
parallel vacuum patch so that it checks the participation of indexes
and then vacuums on un-participated indexes after parallel vacuum.

Regards,

--
Masahiko Sawada