Re: [PATCHES] VACUUM Improvements - WIP Patch

I assume there is no TODO here.

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Pavan Deolasee wrote:
> (taking the discussions to -hackers)
>
> On Sat, Jul 12, 2008 at 11:02 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >
> >
> > (2) It achieves speedup of VACUUM by pushing work onto subsequent
> > regular accesses of the page, which is exactly the wrong thing.
> > Worse, once you count the disk writes those accesses will induce it's
> > not even clear that there's any genuine savings.
> >
>
> Well in the worst case that is true. But in most other cases, the
> second pass work will be combined with other normal activities and the
> overhead will be shared, at least there is a chance for that. I think
> there is a chance for delaying the work until there is any real need
> for that e.g. INSERT or UPDATE on the page which would require a free
> line pointer.
>
>
> > (3) The fact that it doesn't work until concurrent transactions have
> > gone away makes it of extremely dubious value in real-world scenarios,
> > as already noted by Simon.
> >
>
> If there are indeed long running concurrent transactions, we won't get
> any benefit of this optimization. But then there are several more
> common cases of very short concurrent transactions. In those cases and
> for very large tables, reducing the vacuum time is a significant win.
> The FSM will be written early and significant work of the VACUUM can
> be finished quickly.
>
> > It strikes me that what you are trying to do here is compensate for
> > a bad decision in the HOT patch, which was to have VACUUM's first
> > pass prune/defrag a page even when we know we are going to have to
> > come back to that page later. What about trying to fix things so
> > that if the page contains line pointers that need to be removed,
> > the first pass doesn't dirty it at all, but leaves all the work
> > to be done at the second visit? I think that since heap_page_prune
> > has been refactored into a "scan" followed by an "apply", it'd be
> > possible to decide before the "apply" step whether this is the case
> > or not.
> >
>
> I am not against this idea. Just that it still requires us double scan
> of the main table and that's exactly what we are trying to avoid with
> this patch.
>
> Thanks,
> Pavan
>
> --
> Pavan Deolasee
> EnterpriseDB http://www.enterprisedb.com
>
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--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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