Re: Catalog/Metadata consistency during changeset extraction from wal

On Thursday, June 21, 2012 03:56:54 PM Florian Pflug wrote:
> On Jun21, 2012, at 13:41 , Andres Freund wrote:
> > 3b)
> > Ensure that enough information in the catalog remains by fudging the xmin
> > horizon. Then reassemble an appropriate snapshot to read the catalog as
> > the tuple in question has seen it.
> 
> The ComboCID machinery makes that quite a bit harder, I fear. If a tuple is
> updated multiple times by the same transaction, you cannot decide whether a
> tuple was visible in a certain snapshot unless you have access to the
> updating backend's ComboCID hash.
Thats a very good point. Not sure how I forgot that.

It think it might be possible to reconstruct a sensible combocid mapping from 
the walstream. Let me think about it for a while...

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