PostgreSQL Serialisation failure on different ids

Serializable isolation mode can be used to avoid race conditions when upserting equal ids. So for create table u(uid int primary key, name text); if we run two similar transactions T1 and T2: begin isolation level serializable; select * from u where uid = 1; And then continue with for T1 and T2: insert into u (uid, name) values (1, 'A'); After commit; only the first succeeds while the other throws a serialisation failure. This is a very neat feature of the mode to handle unique key violations in complex transactions and not resorting to a specific 'hack' insert ... on conflict. **However** even if uids are different, say uid = 2 and uid = 3, both transactions T1 and T2 **still won't** be able to commit. How could it be possible? Supposedly they create different predicate SIReadlocks and select uses index scan. Where's the trick?