Date: Fri, 17 Nov 2023 16:01:27 -0500
From: Bruce Momjian <bruce@momjian.us>
To: Euler Taveira <euler@eulerto.com>
Cc: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>,
	pgsql-hackers@lists.postgresql.org
Subject: Re: Lifetime of commit timestamps
Message-ID: <ZVfUp4nLj7RnqErM@momjian.us>
References: <20180622.172132.230342845.horiguchi.kyotaro@lab.ntt.co.jp>
 <ZVLDqJE8jCrjmnnP@momjian.us>
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On Fri, Nov 17, 2023 at 03:39:14PM -0300, Euler Taveira wrote:
> On Mon, Nov 13, 2023, at 9:47 PM, Bruce Momjian wrote:
> 
>     Is this documentation change still relevant?
> 
> 
> I think so. AFAICS nothing changed. Unless you read the source code, it is not
> clear that VACUUM removes the information for frozen tuples. They are decoupled
> (but executed in the same routine for convenience), hence, someone can ask why
> the pg_xact_commit_timestamp() returns NULL for a transaction that was executed
> *after* you enable track_commit_timestamp. The answer is the design used a
> existing mechanism to clean up data in order to avoid creating a new one.

Okay, I have developed the attached patch based on Horiguchi-san's
version.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Only you can decide what is important to you.

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diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 93f068edcf..b311a11b33 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -25978,7 +25978,8 @@ SELECT collation for ('foo' COLLATE "de_DE");
     They only provide useful data when the
     <xref linkend="guc-track-commit-timestamp"/> configuration option is
     enabled, and only for transactions that were committed after it was
-    enabled.
+    enabled.  The commit timestamps of frozen tuples are removed during
+    vacuum.
    </para>
 
    <table id="functions-commit-timestamp">

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