Re: [DOC] Document concurrent index builds waiting on each other

On Wed, Mar 25, 2020 at 3:58 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> Hi,
>
> On 2020-03-25 16:30:10 -0300, Alvaro Herrera wrote:
> > I posted this in November
> > https://postgr.es/m/20191101203310.GA12239@alvherre.pgsql but I didn't
> > put time to go through the issues there.
>
> Oh, missed that.
>
>
> > I don't know if my approach is exactly what Andres has in mind
>
> Not quite. I don't think it's generally correct for CIC to set
> PROC_IN_VACUUM. I'm doubtful it's the case even just for plain indexes -
> we don't want rows to be pruned away from under us. I also think we'd
> want to set such a flag during all of the CIC phases?
>
> What I was thinking of was a new flag, with a distinct value from
> PROC_IN_VACUUM. It'd currently just be specified in the
> GetCurrentVirtualXIDs() calls in WaitForOlderSnapshots(). That'd avoid
> needing to wait for other CICs on different relations. Since CIC is not
> permitted on system tables, and CIC doesn't do DML on normal tables, it
> seems fairly obviously correct to exclude other CICs.

That would keep CIC from blocking other CICs, but it wouldn't solve
the problem of CIC blocking vacuum on unrelated tables, right? Perhaps
that's orthogonal though.

James