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

Tomas Vondra wrote:

> AFAIK the stats remain the same within a transaction, and as a function
> runs within a transaction, it will either get new data on the first
> iteration, or it will run all 300 of them. I've checked several
> buildfarm members and I'm yet to see a single duration between 12ms and
> 30sec.

No, there's a call to pg_stat_clear_snapshot() that takes care of that.

> I'm really wondering how that could happen. The only thing that I can
> think of is some strange timing issue, causing lost requests to write
> the stats or maybe some of the stats updates. Hmmm, IIRC the stats are
> sent over UDP - couldn't that be related?

yes, UDP packet drops can certainly happen.  This is considered a
feature (do not cause backends to block when the network socket to stat
collector is swamped; it's better to lose some stat messages instead).

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services