Re: An idle thought

On Thu, 2010-03-18 at 14:48 -0700, Jeff Davis wrote:
> On Thu, 2010-03-18 at 17:17 -0400, Tom Lane wrote:
> > > The VM cause wrong results if a bit is set that's not supposed to be --
> > > right? Am I missing something? How does a seq scan skip visibility
> > > checks and still produce right results, if it doesn't rely on the bit?
> > 
> > It doesn't.  The only thing we currently rely on the VM for is deciding
> > whether a page needs vacuuming
> 
> Oh, my mistake. I misremembered the discussion and I thought the seq
> scan optimization made it in.
> 

Hmm...

>From heapgetpage() in heapam.c, CVS HEAD:
/*                                                                                                              
* If the all-visible flag indicates that all tuples on the page
are                                            
* visible to everyone, we can skip the per-tuple visibility tests.
But                                         

I tested in gdb, and it calls HeapTupleSatisfiesMVCC, until I VACUUM a
few times, and then it doesn't call it any more. So, apparently the seq
scan optimization _is_ there. And that means it is correctness-critical.

Regards,Jeff Davis