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From: Tom Lane <[email protected]>
To: PALAYRET Jacques <[email protected]>
Cc: [email protected]
Subject: Re: PostgreSQL trigger how to detect a column value explicitely modified
Date: Tue, 04 Nov 2025 10:08:09 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>

PALAYRET Jacques <[email protected]> writes:
> 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. 

I believe that an ON UPDATE trigger coded in C can access a bitmapset
that shows which column(s) are targeted in the SET clause; but we've
not exposed that to PL/pgSQL or other higher-level languages.

There are of course a bunch of definitional issues.  Should
"UPDATE ... SET x = x" count as an update?  What if some earlier
(... or later ...) BEFORE trigger changes a column?  We don't
provide any help for those cases either.

I think most people settle for testing "OLD.col IS DISTINCT FROM
NEW.col", which you could argue is a good operational definition
of whether the column changed.

			regards, tom lane

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