Re: Question: Is it possible to get the new xlog position after query execution?

On Sun, Oct 31, 2021 at 4:29 PM Peter J. Holzer <hjp-pgsql(at)hjp(dot)at> wrote:

> On 2021-10-29 13:22:56 -0400, Oleg Serov wrote:
> > We are using a master/slave replication system where we perform writes on
> > master and use replication to offload reads.
> >
> > However, sometimes we have a replication lag of a few seconds and as a
> result,
> > after the update, the change is not yet available on the replica.
> >
> > Is there a way to get XLOG position to which specific update query will
> be
> > written? That way we can check if our replica caught up with changes and
> it is
> > safe to read it from. Can it be done using SQL functions? Can I get that
> > information from query protocol?
>
> I think I would prefer a more direct approach:
>
> If you know what you've written, can't you just check whether the
> replica has the new value(s)?

The simplest answer: One thread on a single process on a server knows about
it. Now another thread on another process/other server does not know about
it.

>
> If not, an alternative could be a table which contains a simple counter
> or timestamp:
>
> begin;
> (lots of updates ...)
> commit;
> begin;
> update counter set c = c + 1 returning c; -- save this as c_current
> commit;
>
> Select c from counter on the replica in a loop until c >= c_current.
>
Why invent something totally new when XLOG position is already used for
replication by postgres? What are the benefits of it?

>
> hp
>
> --
> _ | Peter J. Holzer | Story must make more sense than reality.
> |_|_) | |
> | | | hjp(at)hjp(dot)at | -- Charles Stross, "Creative writing
> __/ | http://www.hjp.at/ | challenge!"
>