Re: Recovering from detoast-related catcache invalidations

This is an interesting idea.
Although some catalog tables are not in catcaches,
such as pg_depend, when scanning them, if there is any
SharedInvalidationMessage, the CatalogSnapshot
will be invalidated and recreated ("RelationInvalidatesSnapshotsOnly"
in syscache.c)
Maybe during the system_scan, it receives the SharedInvalidationMessages
and returns the tuples which
are out of date. systable_recheck_tuple is used in dependency.c for such
case.

Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> 于2024年1月14日周日 03:12写道：

> I wrote:
> > Xiaoran Wang <fanfuxiaoran(at)gmail(dot)com> writes:
> >> Hmm, how about first checking if any invalidated shared messages have
> been
> >> accepted, then rechecking the tuple's visibility?
> >> If there is no invalidated shared message accepted during
> >> 'toast_flatten_tuple',
> >> there is no need to do then visibility check, then it can save several
> >> CPU cycles.
>
> > Meh, I'd just as soon not add the additional dependency/risk of bugs.
> > This is an expensive and seldom-taken code path, so I don't think
> > shaving a few cycles is really important.
>
> It occurred to me that this idea might be more interesting if we
> could encapsulate it right into systable_recheck_tuple: something
> like having systable_beginscan capture the current
> SharedInvalidMessageCounter and save it in the SysScanDesc struct,
> then compare in systable_recheck_tuple to possibly short-circuit
> that work. This'd eliminate one of the main bug hazards in the
> idea, namely that you might capture SharedInvalidMessageCounter too
> late, after something's already happened. However, the whole idea
> only works for catalogs that have catcaches, and the other users of
> systable_recheck_tuple are interested in pg_depend which doesn't.
> So that put a damper on my enthusiasm for the idea.
>
> regards, tom lane
>