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help / color / mirror / Atom feedFrom: Subramanian,Ramachandran <ramachandran.subramanian@alte-leipziger.de>
To: David G. Johnston <david.g.johnston@gmail.com>
Cc: pgsql-novice@lists.postgresql.org <pgsql-novice@lists.postgresql.org>
Subject: AW: Trying to understand Tuple Header
Date: Sun, 3 May 2026 19:25:14 +0000
Message-ID: <b2202bde6eb145bb814806b5606d131a@alte-leipziger.de> (raw)
In-Reply-To: <CAKFQuwawOYYR=Dfa3tWsKOd-baJ-hV7SKMj9FHiMnd9f-=vNAg@mail.gmail.com>
References: <0f8e5544ae8a412bb637fcc3d15f8aaa@alte-leipziger.de>
<CAKFQuwawOYYR=Dfa3tWsKOd-baJ-hV7SKMj9FHiMnd9f-=vNAg@mail.gmail.com>
Thank you so much for your speedy reply.
I Googled and found Egor Rogov’s SQL.
The binary value of t_infomask for both the tuples are identical, but they produce different column values for xmin_commited, xmin_aborted …. in the SQL !!!.
Am I not seeing something that is obvious?
Regards
Ram
SELECT t_ctid, cast(t_infomask as bit(16)) FROM heap_page_items(get_raw_page('One_Page_Wonder',0));
t_ctid | t_infomask
--------+------------------
(0,1) | 0000100000000010
(0,2) | 0000100000000010
(2 rows)
SELECT * FROM One_Page_Wonder ;
id | text | time_stamp
----+--------------------+----------------------------
1 | First row Inserted | 2026-05-03 21:18:24.631047
(1 row)
SELECT '(0,'||lp||')' AS ctid,
CASE lp_flags
WHEN 0 THEN 'unused'
WHEN 1 THEN 'normal'
WHEN 2 THEN 'redirect to '||lp_off
WHEN 3 THEN 'dead'
END AS state,
t_xmin as xmin,
t_xmax as xmax,
(t_infomask & 256) > 0 AS xmin_commited,
(t_infomask & 512) > 0 AS xmin_aborted,
(t_infomask & 1024) > 0 AS xmax_commited,
(t_infomask & 2048) > 0 AS xmax_aborted,
t_ctid
FROM heap_page_items(get_raw_page('One_Page_wonder',0));
ctid | state | xmin | xmax | xmin_commited | xmin_aborted | xmax_commited | xmax_aborted | t_ctid
-------+--------+------+------+---------------+--------------+---------------+--------------+--------
(0,1) | normal | 3673 | 0 | t | f | f | t | (0,1)
(0,2) | normal | 3674 | 0 | f | t | f | t | (0,2)
(2 rows)
Freundliche Grüße
i. A. Ramachandran Subramanian
Zentralbereich Informationstechnologie
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Von: David G. Johnston <david.g.johnston@gmail.com>
Gesendet: Sonntag, 3. Mai 2026 21:03
An: Subramanian,Ramachandran IT-md-db <ramachandran.subramanian@alte-leipziger.de>
Cc: pgsql-novice@lists.postgresql.org
Betreff: Re: Trying to understand Tuple Header
On Sunday, May 3, 2026, Subramanian,Ramachandran <ramachandran.subramanian@alte-leipziger.de<mailto:ramachandran.subramanian@alte-leipziger.de>> wrote:
How does postgres know that this 4th tuple has been rolled back ? Please help me learn that.
The pg_xact file holds information pertaining to whether a transaction produced live or dead tuples upon ending.
David J.
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Subject: Re: AW: Trying to understand Tuple Header
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