Re: Autovacuum fails to keep visibility map up-to-date in mostly-insert-only-tables

On Mon, Oct 20, 2014 at 5:46 PM, Andres Freund <andres(at)2ndquadrant(dot)com>
wrote:

> On 2014-10-20 17:43:26 -0700, Josh Berkus wrote:
> > On 10/20/2014 05:39 PM, Jim Nasby wrote:
> > > Or maybe vacuum isn't the right way to handle some of these scenarios.
> > > It's become the catch-all for all of this stuff, but maybe that doesn't
> > > make sense anymore. Certainly when it comes to dealing with inserts
> > > there's no reason we *have* to do anything other than set hint bits and
> > > possibly freeze xmin.
> >
> > +1
>
> A page read is a page read. What's the point of heaving another process
> do it?

It is only a page read if you have to read the page. It would seem optimal
to have bgwriter adventitiously set hint bits and vm bits, because that is
the last point at which the page can be changed without risking that it be
written out twice. At that point, it has been given the maximum amount of
time it can be given for the interested transactions to have committed and
to have aged past the xmin horizon. I seem to recall that the main problem
with that, though, is that you must be attached to a database in order to
determine visibility, and bgwriter is not attached to a database.

Cheers,

Jeff