Re: Suspicious behaviour on applying XLOG_HEAP2_VISIBLE.

On 2016-04-15 13:07:19 -0400, Robert Haas wrote:
> On Thu, Apr 14, 2016 at 12:11 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On 2016-04-14 11:50:58 -0400, Robert Haas wrote:
> >> On Wed, Apr 13, 2016 at 9:58 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >> > We've recently discussed a very similar issue around
> >> > http://www.postgresql.org/message-id/20160227002958.peftvmcx4dxwe244@alap3.anarazel.de
> >> >
> >> > Unfortunately Simon over in that thread disagreed there about fixing
> >> > this by always emitting a commit record when nmsgs > 0 in
> >> > RecordTransactionCommit(). I think this thread is a pretty strong hint
> >> > that we actually should do so.
> >>
> >> Yes. I'm pretty confident that you had the right idea there, and that
> >> Simon's objection was off-base.
> >
> > The easiest way to achieve that seems to be to just assign an xid if
> > that's the case; while it's not necessarily safe/efficient to do so at
> > the point the invalidation message was queued, I think it should be safe
> > to do so at commit time. Seems less invasive to backpatch than to either
> > support commit records without xids, or a separate record just
> > transporting invalidation messages.
>
> I agree that's better for back-patching.

It's a bit ugly though, since we're at that stage pretty heavily
assuming there's no xid assigned (on the caller level)... I've toyed
with the idea of emitting a commit record that doesn't have an assigned
xid, but that'd require changes on the apply side, which would be uglier
than a new record type :(

> I hope it won't suck performance-wise.

I can't see that be the case, there's not many places where we send
invalidation messages without an assigned xid. Running an instrumented
build with an appropriate wal level reveals about a handfull places.

Greetings,

Andres Freund