Marko Tiikkaja wrote:
> On 8/5/2010 9:44 PM, Merlin Moncure wrote:
> > On Thu, Aug 5, 2010 at 2:09 PM, Tom Lane<[email protected]> wrote:
> >> I was not persuaded that there's a real bug in practice. IMO, his
> >> problem was a broken trigger not broken upsert logic. Even if we
> >> conclude this is unsafe, simply removing the example is of no help to
> >> anyone.
> >
> > Well, the error handler is assuming that the unique_volation is coming
> > from the insert made within the loop. This is obviously not a safe
> > assumption in an infinite loop context. It should be double checking
> > where the error was being thrown from -- but the only way I can think
> > of to do that is to check sqlerrm.
>
> Yeah, this is a known problem with our exception system. If there was
> an easy and reliable way of knowing where the exception came from, I'm
> sure the example would include that.
>
> > Or you arguing that if you're
> > doing this, all dependent triggers must not throw unique violations up
> > the exception chain?
>
> If he isn't, I am. I'm pretty sure you can break every example in the
> docs with a trigger (or a rule) you haven't thought through.
>
> >> A more useful response would be to supply a correct example.
> > Agree: I'd go further I would argue to supply both the 'safe' and
> > 'high concurrency (with caveat)' way. I'm not saying the example is
> > necessarily bad, just that it's maybe not a good thing to be pointing
> > as a learning example without qualifications. Then you get a lesson
> > both on upsert methods and defensive error handling (barring
> > objection, I'll provide that).
>
> The problem with the "safe" way is that it's not safe if called in a
> transaction with isolation level set to SERIALIZABLE.
Good analysis. Documentation patch attached and applied.
--
Bruce Momjian <[email protected]> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ It's impossible for everything to be true. +
diff --git a/doc/src/sgml/plpgsql.sgml b/doc/src/sgml/plpgsql.sgml
index c342916..d2e74dc 100644
*** a/doc/src/sgml/plpgsql.sgml
--- b/doc/src/sgml/plpgsql.sgml
*************** BEGIN
*** 2464,2470 ****
INSERT INTO db(a,b) VALUES (key, data);
RETURN;
EXCEPTION WHEN unique_violation THEN
! -- do nothing, and loop to try the UPDATE again
END;
END LOOP;
END;
--- 2464,2470 ----
INSERT INTO db(a,b) VALUES (key, data);
RETURN;
EXCEPTION WHEN unique_violation THEN
! -- Do nothing, and loop to try the UPDATE again.
END;
END LOOP;
END;
*************** LANGUAGE plpgsql;
*** 2474,2480 ****
SELECT merge_db(1, 'david');
SELECT merge_db(1, 'dennis');
</programlisting>
!
</para>
</example>
</sect2>
--- 2474,2483 ----
SELECT merge_db(1, 'david');
SELECT merge_db(1, 'dennis');
</programlisting>
! This example assumes the <literal>unique_violation</> error is caused by
! the <command>INSERT</>, and not by an <command>INSERT</> trigger function
! on the table. Also, this example only works in the default Read
! Committed transaction mode.
</para>
</example>
</sect2>
--
Sent via pgsql-hackers mailing list ([email protected])
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers
