READ UNCOMMITTED In the READ UNCOMMITTED isolation level, transactions can view the results of un- committed transactions. At this level, many problems can occur unless you really, really know what you are doing and have a good reason for doing it. This level is rarely used in practice, because its performance isn’t much better than the other levels, which have many advantages. Reading uncommitted data is also known as a dirty read. READ COMMITTED The default isolation level for most database systems (but not MySQL!) is READ COMMITTED . It satisfies the simple definition of isolation used earlier: a transaction will see only those changes made by transactions that were already committed when it began, and its changes won’t be visible to others until it has committed. This level still allows what’s known as a nonrepeatable read. This means you can run the same statement twice and see different data. REPEATABLE READ REPEATABLE READ solves the problems that READ UNCOMMITTED allows. It guarantees that any rows a transaction reads will “look the same” in subsequent reads within the same transaction, but in theory it still allows another tricky problem: phantom reads. Simply put, a phantom read can happen when you select some range of rows, another transaction inserts a new row into the range, and then you select the same range again; you will then see the new “phantom” row. InnoDB and XtraDB solve the phantom read problem with multiversion concurrency control, which we ex- plain later in this chapter. REPEATABLE READ is MySQL’s default transaction isolation level. SERIALIZABLE The highest level of isolation, SERIALIZABLE , solves the phantom read problem by forcing transactions to be ordered so that they can’t possibly conflict. In a nutshell, SERIALIZABLE places a lock on every row it reads. At this level, a lot of timeouts and lock contention can occur. We’ve rarely seen people use this isolation level, but your application’s needs might force you to accept the decreased concurrency in favor of the data stability that results.
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