Re: foreign key locks, 2nd attempt

On Thu, Mar 15, 2012 at 08:37:36PM -0400, Robert Haas wrote:
> On Thu, Mar 15, 2012 at 5:07 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> > On Wed, Mar 14, 2012 at 5:23 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> >>> You still have HEAP_XMAX_{INVALID,COMMITTED} to reduce the pressure on mxid
> >>> lookups, so I think something more sophisticated is needed to exercise that
> >>> cost. ?Not sure what.
> >>
> >> I don't think HEAP_XMAX_COMMITTED is much help, because committed !=
> >> all-visible.
> >
> > So because committed does not equal all visible there will be
> > additional lookups on mxids? That's complete rubbish.
>
> Noah seemed to be implying that once the updating transaction
> committed, HEAP_XMAX_COMMITTED would get set and save the mxid lookup.
> But I think that's not true, because anyone who looks at the tuple
> afterward will still need to know the exact xmax, to test it against
> their snapshot.

Yeah, my comment above was wrong. I agree that we'll need to retrieve the
mxid members during every MVCC scan until we either mark the page all-visible
or have occasion to simplify the mxid xmax to the updater xid.