Simon Riggs <simon@2ndQuadrant.com> writes:
> My patch actually improves the speed of snapshots, rather than slowing
> them as Tom's would.
It can be arbitrarily fast if it doesn't have to get the right answer.
Unfortunately, you're not producing the right answer. You can not
exclude XIDs in other databases from the snapshot, at least not unless
you know that the snapshot will not be used for examining any shared
catalogs ... and GetSnapshotData certainly cannot know that.
I think that the idea of computing a different cutoff on the
probably-rare occasions where we need to prune a shared catalog page
has some merit, but the change you are currently proposing to
GetSnapshotData flat out does not work.
regards, tom lane