On 2016-04-15 17:37:03 -0300, Alvaro Herrera 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).

Yea, I remember ;). We made that decision because we couldn't find
another way, and because the consequences were pretty grave.

> 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.

There could, if they're using wal_keep_segments, and the standby cannot
be caught up anymore.

I think it's still worth to go for the new record type, but it's a
pretty close call. We could also just decide to document the issues :/ -
but I'm not sure we're eing all of them yet.

- Andres

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