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

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

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_ | Peter J. Holzer | Story must make more sense than reality.
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