Re: Vacuum, visibility maps and SKIP_PAGES_THRESHOLD

On Fri, May 27, 2011 at 7:36 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Pavan Deolasee <pavan.deolasee@gmail.com> writes:
>> My statistical skills are limited, but wouldn't that mean that for a
>> fairly well distributed write activity across a large table, if there
>> are even 3-4% update/deletes, we would most likely hit a
>> not-all-visible page for every 32 pages scanned ?
>
> Huh?  With a typical table density of several dozen tuples per page, an
> update ratio in that range would mean that just about every page would
> have something for VACUUM to do, if the modified tuples are evenly
> distributed.  The case where the skip optimization has some use is where
> there are large "cold" sections that have no changes at all.
>

I was pretty sure that I would have done my maths wrong :-) So that
means, even for far lesser update ratio, we would pretty much scan
every block and vacuum many of them for a typical well distributed
updates. Hmm. That means the idea of a single pass vacuum is
interesting even after visibility maps.

Thanks,
Pavan

--
Pavan Deolasee
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