Re: CLUSTER and MVCC

Zeugswetter Andreas ADI SD wrote:
>
> > > 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.)

Yes, it does an indexscan (last time I checked, at least). I think if a
performance improvement is demonstrated, we would accept a patch ...

--
Alvaro Herrera http://www.CommandPrompt.com/
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