Re: old_snapshot_threshold bottleneck on replica

Hi,

On 2023-01-24 10:46:28 -0500, Robert Haas wrote:
> On Mon, Jan 23, 2023 at 9:40 AM Maxim Orlov <orlovmg@gmail.com> wrote:
> > One of our customers stumble onto a significant performance degradation while running multiple OLAP-like queries on
areplica.
 
> > After some investigation, it became clear that the problem is in accessing old_snapshot_threshold parameter.
>
> It has been suggested that we remove that feature entirely.

Indeed. There's a lot of things wrong with it. We have reproducers for
creating wrong query results. Nobody has shown interest in fixing the
problems, for several years by now. It costs users that *do not* use the
feature performance (*).

I think we're doing our users a disservice by claiming to have this feature.

I don't think a lot of the existing code would survive if we were to create a
newer version, more maintainable / reliable, version of the feature.

Greetings,

Andres Freund

(*) E.g. TestForOldSnapshot() is called in a good number of places, and emits
    quite a bit of code. It's not executed, but the emitted code is large
    enough to lead to worse code being generated.