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

Hi,

On 2020-04-15 09:31:58 -0400, James Coleman wrote:
> On Wed, Mar 25, 2020 at 3:58 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > 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.

I am not sure what blocking you are referring to here? CIC shouldn't
block vacuum on other tables from running? Or do you just mean that
vacuum will not be able to remove some rows due to the snapshot from the
CIC? That'd be an orthogonal problem, yes.

If it's about the xmin horizon for vacuum: I think we could probably
avoid that using the same flag. As vacuum cannot be run against a table
that has a CIC running (although it'd theoretically be possible to allow
that), it should be safe to ignore PROC_IN_CIC backends in vacuum's
GetOldestXmin() call. That might not be true for system relations, but
we don't allow CIC on those.

Greetings,

Andres Freund