Re: snapshot too old issues, first around wraparound and then more.

Hi,

On 2020-04-13 14:58:34 +1200, Thomas Munro wrote:
> On Fri, Apr 3, 2020 at 2:22 PM Peter Geoghegan <pg@bowt.ie> wrote:
> > I think that it's worth considering whether or not there are a
> > significant number of "snapshot too old" users that rarely or never
> > rely on old snapshots used by new queries. Kevin said that this
> > happens "in some cases", but how many cases? Might it be that many
> > "snapshot too old" users could get by with a version of the feature
> > that makes the most conservative possible assumptions, totally giving
> > up on the idea of differentiating which blocks are truly safe to
> > access with an "old" snapshot? (In other words, one that assumes that
> > they're *all* unsafe for an "old" snapshot.)
> >
> > I'm thinking of a version of "snapshot too old" that amounts to a
> > statement timeout that gets applied for xmin horizon type purposes in
> > the conventional way, while only showing an error to the client if and
> > when they access literally any buffer (though not when the relation is
> > a system catalog). Is it possible that something along those lines is
> > appreciably better than nothing to users? If it is, and if we can find
> > a way to manage the transition, then maybe we could tolerate
> > supporting this greatly simplified implementation of "snapshot too
> > old".
> 
> Hi Peter,
> 
> Interesting idea.  I'm keen to try prototyping it to see how well it
> works out it practice.  Let me know soon if you already have designs
> on that and I'll get out of your way, otherwise I'll give it a try and
> share what I come up with.

FWIW, I think the part that is currently harder to fix is the time->xmin
mapping and some related pieces. Second comes the test
infrastructure. Compared to those, adding additional checks for old
snapshots wouldn't be too hard - although I'd argue that the approach of
sprinkling these tests everywhere isn't that scalable...

Greetings,

Andres Freund