Hi, On 2023-04-14 10:05:08 -0400, Greg Stark wrote: > On Thu, 13 Apr 2023 at 13:01, Peter Geoghegan <p...@bowt.ie> wrote: > > > > On Thu, Apr 13, 2023 at 9:45 AM Robert Haas <robertmh...@gmail.com> wrote: > > > > > > On Wed, Apr 12, 2023 at 4:23 PM Greg Stark <st...@mit.edu> wrote: > > > > Am I crazy or is the parenthetical comment there exactly backwards? If > > > > the horizon is *more recent* then fewer tuples are *non*-removable. > > > > I.e. *more* tuples are removable, no? > > > > > > Isn't it the non-parenthetical part that's wrong? I would expect that > > > if we don't know which relation it is, the horizon might be > > > considerably LESS recent, which would result in fewer tuples being > > > removable.
If rel is *not NULL*, the horizon is more recent - that seems correct? > > You can make arguments for either way of restating it being clearer > > than the other. > > Yeah, I think Robert is being confused by the implicit double > negative. If we don't know which relation it is it's because relation > is NULL and the comment is talking about if it's "not NULL". I think > you're right that it would be less confusing if it just says "if you > pass NULL we have to give a conservative result which means an older > xid and fewer removable tuples". > > But I'm saying the parenthetical part is not just confusing, it's > outright wrong. I guess that just means the first half was so > confusing it confused not only the reader but the author too. I don't think it's outright wrong, but it is very confusing what it relates to. For some reason I tried to "attach" the parenthetical to the "otherwise", which doesn't make a whole lot of sense. How about: * If rel is not NULL the horizon may be considerably more recent (i.e. * allowing more tuples to be removed) than otherwise. In the NULL case a * horizon that is correct (but not optimal) for all relations will be * returned. Thus, if possible, a relation should be provided. Greetings, Andres Freund
