On 2014-01-21 19:45:19 -0500, Tom Lane wrote:
> Andres Freund <and...@2ndquadrant.com> writes:
> > On 2014-01-21 19:23:57 -0500, Tom Lane wrote:
> >> I'm not suggesting that we stop providing that information!  I'm just
> >> saying that we perhaps don't need to store it all in one WAL record,
> >> if instead we put the onus on WAL replay to be able to reconstruct what
> >> it needs from a series of WAL records.
> > That'd likely require something similar to the incomplete actions used
> > in btrees (and until recently in more places). I think that is/was a
> > disaster I really don't want to extend.
> I don't think that's a comparable case.  Incomplete actions are actions
> to be taken immediately, and which the replayer then has to complete
> somehow if it doesn't find the rest of the action in the WAL sequence.
> The only thing to be done with the records I'm proposing is to remember
> their contents (in some fashion) until it's time to apply them.  If you
> hit end of WAL you don't really have to do anything.

Would that work for the promotion case as well? Afair there's the
assumption that everything >= TransactionXmin can be looked up in
pg_subtrans or in the procarray - which afaics wouldn't be the case with
your scheme? And TransactionXmin could very well be below such an
"incomplete commit"'s xids afaics.


Andres Freund

 Andres Freund                     http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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