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Date: Tue, 02 Apr 2024 13:11:27 +0300
From: Kartyshov Ivan <i.kartyshov@postgrespro.ru>
To: Andy Fan <zhihuifan1213@163.com>
Cc: Alexander Korotkov <aekorotkov@gmail.com>, Bharath Rupireddy
 <bharath.rupireddyforpostgres@gmail.com>, Euler Taveira <euler@eulerto.com>,
 Thomas Munro <thomas.munro@gmail.com>, Peter Eisentraut
 <peter@eisentraut.org>, Amit Kapila <amit.kapila16@gmail.com>, Dilip Kumar
 <dilipbalaut@gmail.com>, Peter Smith <smithpb2250@gmail.com>,
 pgsql-hackers@lists.postgresql.org
Subject: Re: [HACKERS] make async slave to wait for lsn to be replayed
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On 2024-04-02 11:14, Andy Fan wrote:
> If so, when users call pg_wal_replay_wait, they can get informed when
> the wal is replaied to the target_lsn, but they can't know how long 
> time
> it waits unless they check it in application side, I think such
> information will be useful for monitor purpose sometimes.
> 
> select pg_wal_replay_wait(lsn, 1000);  may just take 1ms in fact, in
> this case, I want this function return 1.

Hi Andy, to get timing we can use \time in psql.
Here is an example.
postgres=# \timing
Timing is on.
postgres=# select 1;
  ?column?
----------
         1
(1 row)

Time: 0.536 ms


>        <returnvalue>void</returnvalue>
And returning VOID is the best option, rather than returning TRUE|FALSE
or timing. It left the logic of the procedure very simple, we get an
error if LSN is not reached.

8 years, we tried to add this feature, and now we suggest the best way
for this feature is to commit the minimal version first.

Let's discuss further improvements in future versions.

-- 
Ivan Kartyshov
Postgres Professional: www.postgrespro.com