Re: CLUSTER and MVCC

Ühel kenal päeval, R, 2007-03-09 kell 11:29, kirjutas Heikki
Linnakangas:
> Is there a particular reason why CLUSTER isn't MVCC-safe? It seems to me 
> that it would be trivial to fix, by using SnapshotAny instead of 
> SnapshotNow, and not overwriting the xmin/xmax with the xid of the 
> cluster command.
> 
> I feel that I must missing something, or someone would've already fixed 
> it a long time ago...

Probably it is not MVCC safe because the relation is swapped out from
under the pg_class.

That is, it can be possible , that older and newer transactions read
different datafiles and so simle MVCC does not work.


> Csaba, you mentioned recently 
> (http://archives.postgresql.org/pgsql-hackers/2007-03/msg00027.php) that 
> you're actually using the MVCC-violation to clean up tables during a 
> backup. Can you tell us a bit more about that? Would you be upset if we 
> shut that backdoor?
> 
> In any case, the MVCC-violation needs to be documented. I'll send a doc 
> patch to pgsql-patches shortly.
> 
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