Here are some explainations about SYNC:
http://archives.postgresql.org/pgsql-admin/2007-03/msg00109.php


On 7/26/07, Richard Yen <dba at richyen.com> wrote:
>
> > If the origin is not being updated, then it may take 30 seconds to
> > generate a SYNC (depending on configuration!).  In that case,
> > subscribers may sit at the same SYNC number as the origin for that 30
> > seconds.
> Are you referring to the sync_interval_timeout parameter in my conf
> file?  I'm set to sync_interval_timeout=3D3000
>
> I'm also set to sync_interval=3D500.  Is this supposed to create a SYNC
> event every .5 seconds?  Or perhaps the subscriber is supposed to
> look for a SYNC event every .5 seconds?
>
> Perhaps my understanding of SYNC is misshapen, and it's being exposed
> here...
>
> > After 30s, one of the timeouts is reached, a SYNC (that involves no
> > updates!) is generated, and so it takes very little time for the
> > subscribers to process that SYNC, and then sit, lagging by nothing,
> > for another 30s.
> >
> > If the application is quiescent, then that's a to-be-expected
> > behaviour.
> Thanks for the response!
> --Richard
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