I had the same problem some time ago. I wanted to define a trigger firing on CREATE TABLE (pg_class). This won't work because in most cases system tables are not accessed using the "standard" way for processing queries (parse -> rewrite -> plan -> execute). Therefore triggers are not allowed. Triggers could lead to conflicts and corruption. I have done some code digging and I have removed the warning you have shown. It does not help - I guess there is no way to get rid of the problem. All you can do is to write a function doing the job for you. System tables don't care about events too much.
There used to be a whole thread on this subject matter called "Triggers and System Tables" - maybe digging the archives will give you a broader view of the problem.
Too bad - it would be a nice feature :).
In my case I have solved the problem with the help of a view and a set of rules - maybe this will work for you as well. Also, functions are a good choice.
Stef Telford wrote:
First of all, kudos on postgreSQL. That being said, am having a slight problem with it, namely, System tables appear to be 'special tables' and disallow triggers. Perhaps its jst the way i am doing it, however, here is a rather trite example to illustrate (file called system.trig):
CREATE OR REPLACE FUNCTION action_upd() RETURNS opaque AS ' BEGIN RAISE NOTICE ''User % added/updated'',NEW.usename; RETURN new; END;' LANGUAGE 'plpgsql';
CREATE TRIGGER mysys_trig AFTER INSERT OR UPDATE on pg_shadow FOR EACH ROW EXECUTE PROCEDURE action_upd();
test1=# \i /data_raid/stefs_home/postgres/system.trig
psql:/data_raid/stefshome/postgres/system.trig:10: ERROR: CreateTrigger: can't create trigger for system relation pg_shadow
Now, i know that this is largely a useless example, however the theory was that if this -was- possible, then it would be child's play to create a session table for each user. This would allow the system to store values which would make things easier for the rest of the database and apache system i am using (it would allow me to improve and normalise my schema somewhat). I can also see the above being useful for when you have more than one DBA and would like to log user creation (actually, am thinking more of using pl/perl here and then emailing out each user/modification to all admins)
Last time this was tried (around 7.2) it allowed triggers on system tables, but pg_user and pg_shadow and most other tables would have values inserted in outside of the 'INSERT' logic. I can understand that this is great for speed, but it seriously hampers the usefulness of triggers (they wouldnt fire :)
This would seem to be a step backwards to me though, as now we cant even put triggers on the system tables. Is there any chance of this being lifted ? (actually, can understand not allowing pg_trigger mainly because it could end up a cyclic trigger very easily :)
Thanks and hope this makes some sort of sense :)
regards Stef Telford <[EMAIL PROTECTED]>
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