concurrency control (transactions) for Oracle

1, the acid characteristics of the transaction: atomicity atomicity, consistency consistency, isolation isolation, durability persistence.

2, the operation of the transaction

① Transaction Commit sql>commit;   ② Transaction Rollback sql>rollback;     ③ Save point Sql>savepoint save name; Rollback to save the roll call;

Note: I. A transaction that has already been committed cannot be rolled back. II. A transaction that rolls back to a reservation will undo all modifications after the retention point.

④set Transaction statement: Used to set various properties of a transaction.

The ① property has an isolation layer that specifies the transaction ② the storage space used when the transaction is rolled back ③ the named transaction

3. End Transaction

The following conditions Oracle considers a transaction to end:

①commit

②rollback

③ Execute DDL statement (Create, Drop, Alter), transaction is committed

④ disconnecting from Oracle, transaction is committed

⑤ user process is terminated unexpectedly, transaction is rolled back

Simply put, commit and rollback are two ways to end, except that the commit and rollback situations are different.

4. Isolation LEVEL

Through some phenomena, the isolation level effect can be reflected. These phenomena are: ① update lost ② dirty read ③ non-repeatable read ④ illusion

	Dirty Read	Non-REPEATABLE READ	Illusion
Read not submitted	Yes	Yes	Yes
Read commit (default)	No	Yes	Yes
Repeat Read	No	No	Yes
Serialization of	No	No	No

5. Setting the isolation Level

In Oracle, you can set the isolation level of a transaction in the following ways before the transaction begins:

SET transation Isolation Level READ COMMITTED; (Oracle default transaction isolation Levels)

SET transation Isolation Level SERIALIZABLE;

SET transation READ only;

You can set the isolation level for an entire session in a separate session:

ALTER SESSION SET Isolation_lebel SERIALIZABLE;

ALTER SESSION SET Isolation_lebel READ commited;

ALTER SESSION SET Isolation_lebel READ only;

concurrency control (transactions) for Oracle