On 17 August 2017 at 09:33, Andres Freund <and...@anarazel.de> wrote:

> On 2017-08-16 21:25:48 -0400, Robert Haas wrote:
> > On Wed, Aug 16, 2017 at 5:55 PM, Andres Freund <and...@anarazel.de>
> 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.
> >
> > Maybe I shouldn't play the devil's advocate here, but isn't a feature
> > like this by definition only for people who Know What They Are Doing?
> > If so, why not let them back the slot up?  I'm sure that will work out
> > just fine.  They Know What They Are Doing.
> I have yet to hear a reason for allowing to move things backward etc. So
> I'm not sure what the benefit would be. But more importantly I'd like to
> make this available to non-superusers at some point, and there I think
> it's more important that they can't do bad things. The reason for
> allowing it for non-superusers is that I think it's quite a useful
> function to be used by an automated system. E.g. to ensure enough, but
> not too much, WAL is available for a tertiary standby both on the actual
> primary and a failover node.

I strongly agree.

If you really need to move a physical slot back (why?) you can do it with
an extension that uses the low level APIs. But I can't see why you would
want to.

 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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