| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Heikki Linnakangas <heikki(at)enterprisedb(dot)com> |
| Cc: | PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, nagy(at)ecircle-ag(dot)com |
| Subject: | Re: CLUSTER and MVCC |
| Date: | 2007-03-09 15:58:17 |
| Message-ID: | 29752.1173455897@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Heikki Linnakangas <heikki(at)enterprisedb(dot)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
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