Robert Haas wrote:
> On Wed, Jul 6, 2016 at 6:06 PM, Andres Freund <> wrote:
> > Hm. We can't easily do that in the back-patched version; because a
> > standby won't know to check for the flag . That's kinda ok, since we
> > don't yet need to clear all-visible yet at that point of
> > heap_update. But that better means we don't do so on the master either.
> Is there any reason to back-patch this in the first place?

Wasn't this determined to be a pre-existing bug?  I think the
probability of occurrence has increased, but it's still possible in
earlier releases.  I wonder if there are unexplained bugs that can be
traced down to this.

I'm not really following this (sorry about that) but I wonder if (in
back branches) the failure to propagate in case the standby wasn't
updated can cause actual problems.  If it does, maybe it'd be a better
idea to have a new WAL record type instead of piggybacking on lock
tuple.  Then again, apparently the probability of this bug is low enough
that we shouldn't sweat over it ... Moreso considering Robert's apparent
opinion that perhaps we shouldn't backpatch at all in the first place.

In any case I would like to see much more commentary in the patch next
to the new XLHL flag.  It's sufficiently different than the rest than it
deserves so, IMO.

Álvaro Herrera      
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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