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help / color / mirror / Atom feedFrom: Subramanian,Ramachandran <[email protected]>
To: David G. Johnston <[email protected]>
Cc: [email protected] <[email protected]>
Subject: AW: Trying to understand Tuple Header
Date: Mon, 4 May 2026 04:04:30 +0000
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAKFQuwbRKbjk4JPefgT6s64hN-i47a1nZO6O4j-+h48er6uyvw@mail.gmail.com>
References: <[email protected]>
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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
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Von: David G. Johnston <[email protected]>
Gesendet: Sonntag, 3. Mai 2026 21:49
An: Subramanian,Ramachandran IT-md-db <[email protected]>
Cc: [email protected]
Betreff: Re: Trying to understand Tuple Header
On Sun, May 3, 2026 at 12:25 PM Subramanian,Ramachandran <[email protected]<mailto:[email protected]>> 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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