Subject: Re: SELECT MAX(c) FROM (SELECT ... FOR UPDATE) AS foo
From: Markus Bertheau <twanger@bluetwanger.de>
To: Bruno Wolff III <bruno@wolff.to>
Cc: pgsql-sql@postgresql.org
In-Reply-To: <20040817141229.GA15770@wolff.to>
References: <1092740863.2627.17.camel@dicaprio.akademie1.de>
	<20040817141229.GA15770@wolff.to>
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Message-Id: <1092752492.2627.23.camel@dicaprio.akademie1.de>
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Date: Tue, 17 Aug 2004 16:21:32 +0200
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=D0=92 =D0=92=D1=82=D1=80, 17.08.2004, =D0=B2 16:12, Bruno Wolff III =D0=BF=
=D0=B8=D1=88=D0=B5=D1=82:
> > SELECT MAX(position) FROM (SELECT position FROM classes WHERE name =3D
> > 'foo' FOR UPDATE OF classes) AS foo
> >=20
> > It's clear which rows should be locked here, I think.
>=20
> Even if it was allowed, it probably wouldn't be good enough because it wo=
n't
> protect against newly inserted records.

Can you detail an example where this wouldn't be good enough?

In a PL/pgSQL function I'm doing

PERFORM position FROM class_fields WHERE class =3D arg_class_name;
INSERT INTO class_fields (class, field, position) VALUES
(arg_class_name, arg_field_name, (SELECT MAX(position) FROM class_fields
WHERE class =3D arg_class_name));

Is this unsafe?

The question initially arose because I wanted to do something similar to

SELECT INTO var_new_position MAX(position) FROM class_fields WHERE class
=3D arg_class_name FOR UPDATE OF class_fields;

which didn't work.

Thanks

--=20
Markus Bertheau <twanger@bluetwanger.de>