Re: fixing old_snapshot_threshold's time->xid mapping

On Mon, Apr 20, 2020 at 12:29 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
>
> On Mon, Apr 20, 2020 at 6:35 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > On Mon, Apr 20, 2020 at 11:24 AM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> > >
> > > On Sat, Apr 18, 2020 at 9:27 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > > On Sat, Apr 18, 2020 at 11:47 AM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> > > Is this an improvement? I realise that there is still nothing to
> > > actually verify that early pruning has actually happened. I haven't
> > > thought of a good way to do that yet (stats, page inspection, ...).
> >
> > Could we test the early pruning using xid-burn patch? Basically, in
> > xid_by_minute we have some xids with the current epoch. Now, we burns
> > more than 2b xid and then if we try to vacuum we might hit the case of
> > early pruning no. Do you wnated to this case or you had some other
> > case in mind which you wnated to test?
>
> I mean I want to verify that VACUUM or heap prune actually removed a
> tuple that was visible to an old snapshot. An idea I just had: maybe
> sto_using_select.spec should check the visibility map (somehow). For
> example, the sto_using_select.spec (the version in the patch I just
> posted) just checks that after time 00:11, the old snapshot gets a
> snapshot-too-old error. Perhaps we could add a VACUUM before that,
> and then check that the page has become all visible, meaning that the
> dead tuple our snapshot could see has now been removed.

Okay, got your point. Can we try to implement some test functions
that can just call visibilitymap_get_status function internally? I
agree that we will have to pass the correct block number but that we
can find using TID. Or for testing, we can create a very small
relation that just has 1 block?

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com