Robert Haas <robertmhaas@gmail.com> writes:
> 1. snapshot-error-v1.patch introduces a new special snapshot, called
> SnapshotError. In the cases where we set SnapshotNow as a sort of
> default snapshot, this patch changes the code to use SnapshotError
> instead. This affects scan->xs_snapshot in genam.c and
> estate->es_snapshot in execUtils.c. This passes make check-world, so
> apparently there is no code in the core distribution that does this.
> However, this is safer for third-party code, which will ERROR instead
> of seg faulting. The alternative approach would be to use
> InvalidSnapshot, which I think would be OK too if people dislike this
> approach.
FWIW, I think using InvalidSnapshot would be preferable to introducing
a new concept for what's pretty much the same thing.
> With that done, the only remaining uses of SnapshotNow in our code
> base will be in currtid_byreloid() and currtid_byrelname(). So far no
> one on this list has been able to understand clearly what the purpose
> of those functions is, so I'm copying this email to pgsql-odbc in case
> someone there can provide more insight.
I had the idea they were used for a client-side implementation of WHERE
CURRENT OF. Perhaps that's dead code and could be removed entirely?
> If we don't want to risk any change to the semantics, we can (1) grit
> our teeth and keep SnapshotNow around or (2) use an instantaneous MVCC
> snapshot there, and accept that people who have very large connection
> counts and extremely heavy use of those functions may see a
> performance regression.
Of those I'd go for (2); it's unlikely that an extra snapshot would be
visible compared to the query roundtrip overhead. But another attractive
possibility is to make these functions use GetActiveSnapshot(), which
would avoid taking any new snapshot.
regards, tom lane