Re: the big picture for index-only scans

Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)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.

regards, tom lane