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Message-ID: <65d55d32-c9c3-4446-a367-d3cca3282dea@aklaver.com>
Date: Fri, 23 Aug 2024 08:02:56 -0700
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Subject: Re: On a subscriber, why is last_msg_send_time in
 pg_stat_subscription sometimes null?
To: Koen De Groote <kdg.dev@gmail.com>,
 PostgreSQL General <pgsql-general@lists.postgresql.org>
References: <CAGbX52H8dDW8nVC8Zxg0-HFY58fAXd93XuxC83_DPHARZkpepw@mail.gmail.com>
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From: Adrian Klaver <adrian.klaver@aklaver.com>
In-Reply-To: <CAGbX52H8dDW8nVC8Zxg0-HFY58fAXd93XuxC83_DPHARZkpepw@mail.gmail.com>
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On 8/23/24 07:33, Koen De Groote wrote:
> I'm running this query:
> 
> SELECT subname, received_lsn, latest_end_lsn,
> last_msg_send_time, last_msg_receipt_time
> FROM pg_catalog.pg_stat_subscription;
> 
> And "last_msg_send_time" will sometimes be null. All I can find in the 
> documentation is that this value represents: "Send time of last message 
> received from origin WAL sender; NULL for parallel apply workers"
> 
> But I have no context for what parallel apply workers are or why they 
> are NULL sometimes and other times not. >
> Can anyone explain?

The source has that information:

https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/replication/logical/applyparallelworker.c;h=e7f7d4c5e4bd3ff63687cb21b84534a05a3962cc;hb=HEAD

The top part explains the process.

As to the time:

/*
  * There are three fields in each message received by the parallel apply
  * worker: start_lsn, end_lsn and send_time. Because we have updated these
  * statistics in the leader apply worker, we can ignore these fields in the
  * parallel apply worker (see function LogicalRepApplyLoop).
  */

> 
> Regards,
> Koen De Groote

-- 
Adrian Klaver
adrian.klaver@aklaver.com