Re: Visibility Groups

On Thu, Aug 07, 2008 at 01:30:27PM +0100, Gregory Stark wrote:
> "Simon Riggs" <simon@2ndquadrant.com> writes:
> 
> > Currently, we calculate a single OldestXmin across all snapshots on the
> > assumption that any transaction might access any table.
> >
> > I propose creating "Visibility Groups" that *explicitly* limit the
> > ability of a transaction to access data outside its visibility group(s).
> > By default, visibility_groups would be NULL, implying potential access
> > to all tables.
> >
> > Once set, any attempt to lock an object outside of a transactions
> > defined visibility_groups will result in an error:
> >   ERROR attempt to lock table outside of visibility group(s): foo
> >   HINT you need to set a different value for visibility_groups
> > A transaction can only ever reduce or restrict its visibility_groups, it
> > cannot reset or add visibility groups.
> 
> Hm, so backing up a bit from the specific proposed interface, the key here is
> being able to explicitly mark which tables your transaction will need in the
> future?
> 
> Is it always just a handful of heavily updated tables that you want to
> protect? In that case we could have a lock type which means "I'll never need
> to lock this object". Then a session could issue "LOCK TABLE foo IN
> INACCESSIBLE MODE" or something like that. That requires people to hack up
> their pg_dump or replication script though which might be awkward.
> 
> Perhaps the way to do that would be to preemptively take locks on all the
> objects that you'll need, then have a command to indicate you won't need any
> further objects beyond those. 

+1

-dg
-- 
David Gould       daveg@sonic.net      510 536 1443    510 282 0869
If simplicity worked, the world would be overrun with insects.