9.1/9.2 SERIALIZABLE: expected serialization failure between INSERT and SELECT not encountered

Hi all

In this SO question:

http://dba.stackexchange.com/questions/26905/how-do-i-implement-insert-if-not-found-for-transactions-at-serializable-isolatio/26909#26909

the author is running a series of queries that I'd expect to abort on
commit with a serialisation failure. No such failure occurs, and I'm
wondering why.

SETUP

create table artist (id serial primary key, name text);

SESSION 1 SESSION 2

BEGIN ISOLATION LEVEL SERIALIZABLE;

BEGIN ISOLATION LEVEL
SERIALIZABLE;

SELECT id FROM artist
WHERE name = 'Bob';

INSERT INTO artist (name)
VALUES ('Bob')

INSERT INTO artist (name)
VALUES ('Bob')

COMMIT; COMMIT;

I'd expect one of these two to abort with a serialization failure and
I'm not sure I understand why they don't in 9.1/9.2's new serializable
mode. Shouldn't the SELECT for "Bob" cause the insertion of "Bob" in the
other transaction to violate serializability?

--
Craig Ringer