Re: Set visibility map bit after HOT prune

On Wednesday, December 19, 2012, Robert Haas wrote:

> On Wed, Dec 19, 2012 at 12:26 PM, Pavan Deolasee
> <pavan.deolasee@gmail.com <javascript:;>> wrote:
>
> > I would like to run some pgbench tests where we get the system in a
> > steady state such as all/most updates are HOT updates (not entirely
> > unlikely scenario for many real life cases). And then try running some
> > concurrent queries which can be executed via IOS. My gut feel is that,
> > today we will see slow and continuous drop in performance for these
> > queries because IOS will slowly stop working.
>
> If there are no vacuums, I agree.
>

If the table is randomly updated over its entire size, then pretty much
every block will be not-all-visible (and so disqualified from IOS) before
you hit the default 20% vacuum threshold.  I wonder if there ought not be
another vac threshold, based on vm density rather than estimated obsolete
tuple density.

Cheers,

Jeff