Re: Use of snapshot in logical replication

On 2017-06-08 03:14:55 +0200, Petr Jelinek wrote:
> On 08/06/17 03:08, Craig Ringer wrote:
> > On 7 June 2017 at 18:16, sanyam jain <sanyamjain22(at)live(dot)in> wrote:
> >> Hi,
> >>
> >> Can someone explain the usage of exporting snapshot when a logical
> >> replication slot is created?
> >
> > It's used to pg_dump the schema at a consistent point in history where
> > all xacts are known to be in the snapshot (and thus dumped) or known
> > not to be (so they'll be streamed out on the slot).
> >
>
> Not just schema, it can also be used to get existing data at consistent
> point in history so that changes will follow without gaps or duplicates.
>
> That being said, the built-in logical replication isn't using the
> exported snapshots at all.

Just because it uses them in the same session doesn't really change the
picture, that's more or less just an optimization. It's essentially
just a cheaper version of importing a snapshot that'd have to be
exported in same session.

- Andres