August 2001
Intermediate to advanced
400 pages
9h 28m
English
Content preview from Java Programming with Oracle SQLJ
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9.2. Transaction Isolation Levels
The degree to which the results of one transaction are visible to other transactions is known as the transaction isolation level. You can set the isolation level on a per-transaction basis. The following four levels are defined in the SQL standard:
SERIALIZABLE
-
This is the default specified by the SQL standard. Dirty reads, non-repeatable reads, and phantom reads are not permitted in this isolation level. SERIALIZABLE is the most "strict" of the isolation levels.
REPEATABLE READ
-
Dirty reads and non-repeatable reads are not permitted, but phantom reads are permitted. This isolation level is not supported by Oracle.
READ COMMITTED
-
This is the default used by Oracle. Dirty reads are not permitted, but non-repeatable reads and phantom reads are permitted.
READ UNCOMMITTED
-
Dirty reads, non-repeatable reads, and phantom reads are permitted. This isolation level is not supported by Oracle.
Oracle's default isolation level of READ COMMITTED is acceptable for most programs. Your other choice is SERIALIZABLE, but you should use it only if you have an extremely good reason, as it may slow the performance of your SQL statements. You may change the transaction isolation level using the SQL SET TRANSACTION statement.
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