PostgreSQL trigger how to detect a column value explicitely modified

Hello,

In a trigger body, is there a simple way to know if a column value has been explicitely modified ?
Explicitely modified ; in others words, typically indicated in the SET clause of the UPDATE.
A simple way ; I mean without analysing the SQL statement which can be very difficult according to the statement.

# My trigger function executed by trigger BEFORE UPDATE ON a table tb (with columns id, c2, c3 ; of integer type) FOR EACH ROW :
CREATE OR REPLACE FUNCTION func_tg_upd()
 RETURNS trigger  LANGUAGE plpgsql
AS $function$
BEGIN
  if NEW.c2 != 1
  then
    NEW.c2 := 0 ;
  end if;
return NEW;
END;
$function$
;

# Actions of the trigger
UPDATE tb
SET c2=2
WHERE ...     -- old c2 value can be 2 or another value
;
=> c2 will be changed to 0 by the trigger, OK, because I explicitely indicated the new value of c2 to a value different from 1.
The the old value of c2 could be different or not.

Example :

TABLE tb ;
 id  | c2 | c3
-----+----+----
 100 |  1 |  2

UPDATE tb
SET c2=2
;
 TABLE tb ;
 id  | c2 | c3
-----+----+----
 100 |  0 |  2

---------- ---------- ---------- ---------- ----------

UPDATE tb
SET c3=3
WHERE ...     -- and c2=2 (previous value of c2 is 2)
;
=> c2 will be changed to 0 by the trigger, but I don't want that action (NOT OK for me) because I didn't explicitely indicate the new value 2 to c2.
Example :

TABLE tb ;
 id  | c2 | c3
-----+----+----
 100 |  2 |  2

UPDATE tb
SET c3=3
;
TABLE tb ;
 id  | c2 | c3
-----+----+----
 100 |  0 |  3

Regards

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PALAYRET Jacques
DCSC/GDC
jacques.palayret@meteo.fr
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