Isolation Level

http://www.postgresql.org/docs/9.1/static/transaction-iso.html 講的不錯。Isolation Level 大原則是共通的，只是每一家做的不太一樣。In PostgreSQL, phantom reads are not possible in the implementation of Repeatable Read.

以 Microsoft SQL Server 2005 來說，她多一種 isolation level: snapshot. (PostgreSQL 也有 snapshot isolation.) The main reason for its adoption is that it allows better performance than serializability. 不難想像 serializable 採用 pessimistic concurrency control，而 snapshot 採用 optimistic concurrency control.