Re: Postgres 11 release notes

On 2018-05-11 15:11:31 -0400, Bruce Momjian wrote:
> On Fri, May 11, 2018 at 04:00:58PM -0300, Alvaro Herrera wrote:
> > Bruce Momjian wrote:
> >
> > > OK, so what is the text that people will understand? This?
> > >
> > > Prevent manual VACUUMs on append-only tables from performing
> > > needless index scans
> >
> > Make vacuum cheaper by avoiding scans of btree indexes when not
> > necessary
> > ?
> >
> > Why "manual vacuum"? It's a problem with vacuums invoked from
> > autovacuum too.
>
> Uh, from the commit it says:
>
> When workload on particular table is append-only, then autovacuum
> isn't intended to touch this table. However, user may run vacuum
> manually in order to fill visibility map and get benefits of
> --------
> index-only scans. Then ambulkdelete wouldn't be called for
> indexes of such table (because no heap tuples were deleted), only
> ---------------------------
> amvacuumcleanup would be called In this case, amvacuumcleanup
> would perform full index scan for two objectives: put recyclable
> pages into free space map and update index statistics.
>
> Why would autovacuum run on a table with no expired index entries?

Anti-Wraparound is one case. One where it's really painful to take
forever on lots of unchanged tables. Lots of hot updates another.

Btw, is it just me, or do the commit and docs confuse say stalled when
stale is intended?

> Personally, I think the fact that autovacuum doesn't run on suvch tables
> and therefore doesn't automatically do index-only scans is a problem. I
> tried to fix it years ago, but failed and gave up.

I'm really unhappy about that too.

Greetings,

Andres Freund