Re: New vacuum option to do only freezing

Hi,

On 2019-05-02 10:20:25 -0400, Robert Haas wrote:
> On Tue, Apr 16, 2019 at 12:44 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > So after thinking about this a bit more ...
> > 2. Act as if the tuple were still live, just as would've happened if the
> > state didn't change till just after we looked instead of just before.
> >
> > Option #2 is a lot simpler and safer, and can be implemented as I
> > suggested earlier, assuming we're all good with the assumption that
> > heap_prepare_freeze_tuple isn't going to do anything bad.
> 
> After studying this more carefully, I agree.  You and Andres and
> Alvaro are all correct, and I'm plain wrong.  Thanks for explaining.
> I have committed a patch that changes the logic as per your
> suggestion, and also removes nleft_dead_tuples and nleft_dead_itemids.

It'd be good if somebody could make a pass over the safety mechanisms in
heap_prepare_freeze_tuple(). I added them at some point, after a data
corrupting bug related to freezing, but they really were more intended
as a secondary layer of defense, rather than the primary one.  My
understanding is still that we assume that we never should reach
heap_prepare_freeze_tuple() for something that is below the horizon,
even after this change, right?

Greetings,

Andres Freund