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List:       postgresql-general
Subject:    Re: Question: Is it possible to get the new xlog position after query execution?
From:       "Peter J. Holzer" <hjp-pgsql () hjp ! at>
Date:       2021-10-31 20:29:33
Message-ID: YX78rSSOdFuyL9yi () hjp ! at
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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)?

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.

        hp

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

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