Re: standby apply lag on inactive servers

On 2020/01/31 22:40, Alvaro Herrera wrote:
> On 2020-Jan-31, Fujii Masao wrote:
> 
>> You're thinking to apply this change to the back branches? Sorry
>> if my understanding is not right. But I don't think that back-patch
>> is ok because it changes the documented existing behavior
>> of pg_last_xact_replay_timestamp(). So it looks like the behavior
>> change not a bug fix.
> 
> Yeah, I am thinking in backpatching it.  The documented behavior is
> already not what the code does.

Maybe you thought this because getRecordTimestamp() extracts the
timestamp from even WAL record of a restore point? That is, you're
concerned about that pg_last_xact_replay_timestamp() returns the
timestamp of not only commit/abort record but also restore point one.
Right?

As far as I read the code, this problem doesn't occur because
SetLatestXTime() is called only for commit/abort records, in
recoveryStopsAfter(). No?

>  Do you have a situation where this
> change would break something?  If so, can you please explain what it is?

For example, use the return value of pg_last_xact_replay_timestamp()
(and also the timestamp in the log message output at the end of
recovery) as a HINT when setting recovery_target_time later.
Use it to compare with the timestamp retrieved from the master server,
in order to monitor the replication delay.

Regards,

-- 
Fujii Masao
NTT DATA CORPORATION
Advanced Platform Technology Group
Research and Development Headquarters