From: | Michael Fuhr <mike(at)fuhr(dot)org> |
---|---|
To: | hubert depesz lubaczewski <depesz(at)gmail(dot)com> |
Cc: | fmiddleton(at)verizon(dot)net, pgsql-admin(at)postgresql(dot)org |
Subject: | Re: freezing a particular field in a table |
Date: | 2005-11-23 17:55:22 |
Message-ID: | 20051123175522.GA3828@winnie.fuhr.org |
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Thread: | |
Lists: | pgsql-admin |
On Wed, Nov 23, 2005 at 10:28:01AM +0100, hubert depesz lubaczewski wrote:
> IF
> (NEW.some_field IS NULL AND OLD.some_field IS NOT NULL)
> OR
> (NEW.some_field IS NOT NULL AND OLD.some_field IS NULL)
> OR
> (NEW.some_field IS NOT NULL AND OLD.some_field IS NOT NULL
> AND NEW.some_field <> OLD.some_field)
> THEN
A more succinct expression would be:
IF NEW.some_field IS DISTINCT FROM OLD.some_field THEN
IS DISTINCT FROM is like <> except that it treats NULL as an ordinary
value.
> CREATE TRIGGER impossible_to_change BEFORE UPDATE ON some_table FOR EACH ROW
> EXECUTE PROCEDURE impossible_to_change();
If the table has other triggers and you don't want them to modify
the value either, then the check should probably go in an AFTER
trigger.
--
Michael Fuhr
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