AW: Trying to understand Tuple Header - Mailing list pgsql-novice

From Subramanian,Ramachandran
Subject AW: Trying to understand Tuple Header
Date
Msg-id 544e6b78efba44e5b004a12a8460aa12@alte-leipziger.de
Whole thread
In response to Re: Trying to understand Tuple Header  ("David G. Johnston" <david.g.johnston@gmail.com>)
Responses Re: AW: Trying to understand Tuple Header
List pgsql-novice

Wow!!!  Thank you.  You are correct. T_infomask did change.

 

I began with postgres in Nov 2025 and I am having so much fun learning it. I am deeply grateful for this community.

 

 

So the next select to the table cross references the pg_xact file to check if hint bits need to be updated or not ?  How deep does the next select look into the pg_xact file ? For example if I roll back and never touch this table for 30  Minutes and I come back and do a second select…will it look back into 30 mins of history in pg_xact ?

 

 

It is a wonderful morning to have learned a little bit of PG Magic 😊 .  Thank you all.

 

 

 

 

 

SELECT t_ctid,t_infomask, cast(t_infomask as bit(16))  FROM heap_page_items(get_raw_page('One_Page_Wonder',0));

t_ctid | t_infomask |    t_infomask

--------+------------+------------------

(0,1)  |       2050 | 0000100000000010

(0,2)  |       2050 | 0000100000000010

(2 rows)

 

SELECT * FROM One_Page_Wonder ;

id |        text        |         time_stamp

----+--------------------+----------------------------

  1 | First row Inserted | 2026-05-04 05:50:32.540905

(1 row)

 

SELECT t_ctid,t_infomask, cast(t_infomask as bit(16))  FROM heap_page_items(get_raw_page('One_Page_Wonder',0));

t_ctid | t_infomask |    t_infomask

--------+------------+------------------

(0,1)  |       2306 | 0000100100000010

(0,2)  |       2562 | 0000101000000010

(2 rows)

 

 

Freundliche Grüße

i. A. Ramachandran Subramanian

Zentralbereich Informationstechnologie

Alte Leipziger Lebensversicherung a. G.

Hallesche Krankenversicherung a. G.

______________________

ALH Gruppe
Alte Leipziger-Platz 1, 61440 Oberursel
Tel: +49 (6171) 66-4882
Fax: +49 (6171) 66-800-4882
E-Mail: ramachandran.subramanian@alte-leipziger.de
www.alte-leipziger.de
www.hallesche.de

Alte Leipziger Lebensversicherung a. G., Alte Leipziger-Platz 1, 61440 Oberursel

Vors. des Aufsichtsrats: Dr. Walter Botermann · Vorstand: Christoph Bohn (Vors.), Dr. Jürgen Bierbaum (stv. Vors.), Frank Kettnaker, Dr. Jochen Kriegmeier, Alexander Mayer, Christian Pape, Wiltrud Pekarek, Udo Wilcsek

Sitz Oberursel (Taunus) · Rechtsform VVaG · Amtsgericht Bad Homburg v. d. H. HRB 1583 · USt.-IdNr. DE 114106814

Hallesche Krankenversicherung a. G., Löffelstraße 34-38, 70597 Stuttgart

Vors. des Aufsichtsrats: Dr. Walter Botermann · Vorstand: Christoph Bohn (Vors.), Dr. Jürgen Bierbaum (stv. Vors.), Frank Kettnaker, Dr. Jochen Kriegmeier, Alexander Mayer, Christian Pape, Wiltrud Pekarek, Udo Wilcsek

Sitz Stuttgart · Rechtsform VVaG · Amtsgericht Stuttgart HRB 2686 · USt.-IdNr. DE 147802285

Beiträge zu privaten Kranken- und Pflegekrankenversicherungen unterliegen nicht der Versicherungsteuer (§ 4 (1) Nr. 5 b VersStG) · Versicherungsleistungen sowie Umsätze aus Versicherungsvertreter-/Maklertätigkeiten sind umsatzsteuerfrei

Pflichtangaben der ALH Gruppe gemäß § 35a GmbHG bzw. § 80 AktG

Von: David G. Johnston <david.g.johnston@gmail.com>
Gesendet: Sonntag, 3. Mai 2026 21:49
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 Sun, May 3, 2026 at 12:25 PM Subramanian,Ramachandran <ramachandran.subramanian@alte-leipziger.de> wrote:

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?

 

You didn't re-check the infomask data after running the select query, instead assuming the bits didn't change.  They did.  SELECT is not a read-only operation, it participates in optimizations.  Called "writing hint bits".  Manually evaluating the various tests against the data you did show would have proven that the pre-select-data had zeros where the second query claims there are ones.

 

David J.

 

pgsql-novice by date:

Previous
From: Tom Lane
Date:
Subject: Re: Trying to understand Tuple Header
Next
From: Laurenz Albe
Date:
Subject: Re: AW: Trying to understand Tuple Header