Re: Read Uncommitted

On Mon, 2008-05-26 at 16:55 +0200, Peter Eisentraut wrote:
> Am Montag, 26. Mai 2008 schrieb Simon Riggs:
> > At the moment, a long running SQL Statement can prevent xmin moving
> > forward, which can result in VACUUM and HOT not being able to remove row
> > versions effectively. My proposal is that a Read Uncommitted transaction
> > would not prevent row removal, which then offers no guarantee that the
> > "correct" answer would be returned. Which is *exactly* what that
> > transaction isolation level was designed for.
> >
> > In many cases, an application designer may be able to tell that a
> > particular query will always return the correct answer. For example, we
> > may query against data which is known not to change, even though other
> > data in the same database cluster may be subject to frequent change.
> > e.g. queries against large insert-only tables.
> 
> If the data in a table never changes, why would VACUUM or HOT need to touch 
> it?  The use case isn't clear to me.

I guess the use-case is about a long read-write transaction doing
read-only access to an update-only table and thus blocking vacuum on
other tables.

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Hannu