Re: MVCC catalog access

On 2013-05-22 23:05:40 -0400, Robert Haas wrote:
> On Wed, May 22, 2013 at 11:02 PM, Andres Freund <andres@2ndquadrant.com> wrote:
> > On 2013-05-22 22:51:13 -0400, Robert Haas wrote:
> >> In discussions today, Tom Lane suggested testing the time to start up
> >> a backend and run a simple query such as "SELECT 2+2" in the absence
> >> of a relcache file.
> >
> >> I did this and can't measure any overhead as a result of MVCC catalog
> >> access.  I tried it with no active connections.  I tried it with 600
> >> idle active connections (to make taking MVCC snapshots more
> >> expensive).
> >
> > Did you try it with the 600 transactions actually being in a transaction
> > and having acquired a snapshot?
> 
> No... I can hack something up for that.

Make that actually having acquired an xid. We skip a large part of the
work if a transaction doesn't yet have one afair. I don't think the mere
presence of 600 idle connections without an xid in contrast to just
having max_connection at 600 should actually make a difference in the
cost of acquiring a snapshot?

Greetings,

Andres Freund

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