Lifetime of commit timestamps

Hello.

I don't find any description in the documentation about the
guaranteed lifetime of commit timestamps. I think they are
preserved until corresponding xid goes beyond the freeze horizen,
even though they are actually preserved longer for several
reasons.

If it is not, I think such description is required in
pg_xact_commit_timestamp().

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center


diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 5dce8ef178..633e488cec 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -18029,7 +18029,8 @@ SELECT collation for ('foo' COLLATE "de_DE");
     These functions mainly provide information about when the transactions
     were committed. They only provide useful data when
     <xref linkend="guc-track-commit-timestamp"/> configuration option is enabled
-    and only for transactions that were committed after it was enabled.
+    and only for transactions that were committed after it was enabled. Commit
+    timestamps for frozen tuples are removed at vacuum time.
    </para>
 
    <table id="functions-commit-timestamp">