Re: CLUSTER and MVCC

Heikki Linnakangas <heikki@enterprisedb.com> writes:
> 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.

The reason it's not trivial is that you also have to preserve the t_ctid
links of update chains.  If you look into VACUUM FULL, a very large part
of its complexity is that it moves update chains as a unit to make that
possible.  (BTW, I believe the problem Pavan Deolasee reported yesterday
is a bug somewhere in there --- it looks to me like sometimes the same
update chain is getting copied multiple times.)
        regards, tom lane