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
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Fax: +49 (6171) 66-800-4882
E-Mail: ramachandran.subramanian@alte-leipziger.de
www.alte-leipziger.de
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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.
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