Kent Tong <kent@xxxxxxxxxxxx> writes: > 1: T1 sets isolation to serializable & begins a transaction > 2: T2 sets isolation to serializable & begins a transaction > 3: T1 reads X into v1 > 4: T2 reads Y into v2 > 5: T1 writes v1 into Y > 6: T2 writes v2 into X > 7: T1 commits > 8: T2 commits > Obviously, this sequence is also not a serializable execution. However, it > is allowed by > PostgreSQL. Moreover, according to the MVCC reference above, step 5 should > really > fail because the read timestamp of Y is that of T2, which is greater than > that of T1. If you want that to fail, use a SELECT FOR UPDATE at steps 3/4. My interpretation of MVCC is that the above example isn't even meaningful, because it assumes that "writing into Y" is an overwrite, which it is not in Postgres --- that is, if T2 reads Y again, it'll get the same value as before. regards, tom lane
