Re: Suspicious behaviour on applying XLOG_HEAP2_VISIBLE.

On Sat, Apr 16, 2016 at 5:37 AM, Alvaro Herrera
<alvherre@2ndquadrant.com> wrote:
> Andres Freund wrote:
>> On 2016-04-15 15:26:17 -0400, Tom Lane wrote:
>> > I think the bottom line is that we misdesigned the WAL representation
>> > by assuming that this sort of info could always be piggybacked on a
>> > transaction commit record.  It's time to fix that.
>>
>> I think we got to piggyback it onto a commit record, as long as there's
>> one. Otherwise it's going to be more complex (queuing messages when
>> reading an inval record) and slower (more wal records).  I can easily
>> develop a patch for that, the question is what we do on the back
>> branches...
>
> We have introduced new wal records in back branches previously --
> nothing new (c.f. 8e9a16ab8f7f0e5813644975cc3f336e5b064b6e).  The user
> just needs to make sure to upgrade the standbys first.  If they don't,
> they would die upon replay of the first such record, which they can take
> as an indication that they need to be upgraded; the standby is down for
> some time, but there is no data loss or corruption.

Yeah, introducing a new WAL record to address this issue in
back-branches would not be an issue, and that's what we should do. For
HEAD, let's add that in the commit record.
-- 
Michael