Re: the big picture for index-only scans

Tom Lane wrote:
> Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> writes:
> > On 10.05.2011 20:15, Simon Riggs wrote:
> >> I can picture that. Regrettably, I can also picture the accesses to
> >> the visibility map, the maintenance operations on the VM that are
> >> needed for this and the contention that both of those will cause.
> 
> > I agree that we need to do tests to demonstrate that there's a gain from 
> > the patch, once we have a patch to test. I would be very surprised if 
> > there isn't, but that just means the testing is going to be easy.
> 
> I think Simon's point is that showing a gain on specific test cases
> isn't a sufficient argument.  What we need to know about this sort of
> change is what is the distributed overhead that is going to be paid by
> *everybody*, whether their queries benefit from the optimization or not.
> And what fraction of real-world queries really do benefit, and to what
> extent.  Isolated test cases (undoubtedly chosen to show off the
> optimization) are not adequate to form a picture of the overall cost and
> benefit.

Yes, I assume we are going to need the same kind of tests we did for
other invasive patches like serializable isolation level and hot
standby.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + It's impossible for everything to be true. +