Alvaro Herrera <> writes:
> Andres Freund wrote:
>> I think we should constrain the API to only allow later LSNs than
>> currently in the slot, rather than arbitrary ones. That's why I was
>> thinking of "forward".  I'm not convinced it's a good / safe idea to
>> allow arbitrary values to be set.

> Hmm.  In terms of safety, it is safe to move the LSN backwards, as long
> as the oldest LSN across all slots is not changed -- in other words, the
> actual safe limit is the oldest of all slot LSNs, rather than the
> current position of the slot being manipulated (which is what you're
> saying).

True, but you'd need to take some kind of global lock to verify that,
whereas "move forward only" would only need to examine/change the one

                        regards, tom lane

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