Andres Freund wrote:
> On 2017-08-16 17:06:42 -0400, Tom Lane wrote:

> > If I understand what this is meant to do, maybe better
> > pg_move_replication_slot_lsn() or pg_change_replication_slot_lsn() ?
> > The point being that you're adjusting the LSN pointer contained
> > in the slot, which is distinct from the slot itself.
> 
> 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).

I don't know if this is useful for the use case Magnus described; TBH I
didn't understand that use case.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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