| From: | "Zeugswetter Andreas ADI SD" <ZeugswetterA(at)spardat(dot)at> |
|---|---|
| To: | "Simon Riggs" <simon(at)2ndquadrant(dot)com>, "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 13:10:07 |
| Message-ID: | E1539E0ED7043848906A8FF995BDA57901D43044@m0143.s-mxs.net |
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| Thread: | |
| Lists: | pgsql-hackers |
> > 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.
>
> It's trivial to fix now in this way, but it would break HOT,
> since an indexscan only returns one row per index entry.
Well, with SnapshotAny HOT should probably return all possibly visible
tuples
with an indexscan. (Btw, does CLUSTER really do an index scan ? Seems
for reading a whole table a seq scan and sort is usually cheaper, at
least when the clustering is so bad that a CLUSTER is needed.)
Andreas
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