Isolation level in RDBMS

In the famous ACID abbreviation, I means Isolation (Isolation level). According to wikipedia, isolation is a property that defines how/when the changes made by one operation become visible to other concurrent operations.

To understand isolations level better, lets review the main colitions that may happend in concurrent
environment:

• lost update - A second transaction writes a second value of a data on top of a first value written by a first concurrent transaction, and the first value is lost to other transactions running concurrently which need, by their precedence, to read the first value.
• dirty read - insert or update data by transaction that will be rollbacked further
• non-repeatable read - first transaction read data, then read them again and the same data is modified
• phantom reads - on the second read in transaction, new data is appearing

So, now that's easy to describe isolation levels with the following table:

Isolation level	JDBC equivalent	Dirty reads	Non-repeatable reads	Phantoms	Lost update
None	TRANSACTION_NONE	may occur	may occur	may occur	may occur
Read Uncommitted	TRANSACTION_READ_UNCOMMITTED	may occur	may occur	may occur	-
Read Committed	TRANSACTION_READ_COMMITTED	-	may occur	may occur	-
Repeatable Read	TRANSACTION_REPEATABLE_READ	-	-	may occur	-
Serializible	TRANSACTION_SERIALIZABLE	-	-	-	-

In Java world you can set up any of them on JDBC driver, however there is not guarantee that this level will be applied! Test your database programmatically using the supportsTransactionIsolationLevel() method in java.sql.DatabaseMetaData to be sure.

RDBMSs are supporting isolation levels according to the following table:

Isolation level	MySQL (InnoDb)	Oracle	DB2	H2	Postgres
Read Uncommitted	+	-	+	+	+
Read Committed	+	+	+	+	+
Repeatable Read	+	+	+	-	+
Serializible	+	+	+	+	+

More about transactions in MySQL
More about transactions in Oracle
More about transaction isolation level in IBM DB2
More about transaction in H2
More about transactions in Postgres