Re: Skip all-visible pages during second HeapScan of CIC

On 2017-03-07 21:38:40 -0500, Robert Haas wrote:
> > I wonder however, if careful snapshot managment couldn't solve this as
> > well. I have *not* thought a lot about this, but afaics we can easily
> > prevent all-visible from being set in cases it'd bother us by having an
> > "appropriate" xmin / registered snapshot.
>
> Yeah, but that's a tax on the whole system.

I'm not sure I can buy that argument. CIC *already* holds a snapshot
during each of the two scans. By reducing the amount of time that's held
in the second scan, the systemwide impact is reduced, because it's held
for a shorter amount of time. We need to hold a slightly "more
aggressive" snapshot, that's true, but if you have high xid throughput
those effects should roughly balance each other.

Greetings,

Andres Freund