Re: PATCH: Split stats file per database WAS: autovacuum stress-testing our system

First of all, big thanks for working on this patch and not only
identifying the issues but actually fixing them.

On 14.2.2013 20:23, Alvaro Herrera wrote:
> Here's a ninth version of this patch. (version 8 went unpublished). I
> have simplified a lot of things and improved some comments; I think I
> understand much of it now. I think this patch is fairly close to
> committable, but one issue remains, which is this bit in
> pgstat_write_statsfiles():
>
...

>
> The problem here is that creating these dummy entries will cause a
> difference in autovacuum behavior. Autovacuum will skip processing
> databases with no pgstat entry, and the intended reason is that if
> there's no pgstat entry it's because the database doesn't have enough
> activity. Now perhaps we want to change that, but it should be an
> explicit decision taken after discussion and thought, not side effect
> from an unrelated patch.

I don't see how that changes the autovacuum behavior. Can you explain
that a bit more?

As I see it, with the old (single-file version) the autovacuum worker
would get exacly the same thing, i.e. no stats at all.

Which is exacly what autovacuum worker gets with the new code, except
that the check for last statfile timestamp uses the "per-db" file, so we
need to write it. This way the worker is able to read the timestamp, is
happy about it because it gets a fresh file although it gets no stats later.

Where is the behavior change? Can you provide an example?

kind regards
Tomas