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Managing Transactions

A transaction is a customized sequence of database operations, which form an integral unit of work. In MogDB, you can start, set, commit, and roll back transactions. MogDB supports the READ COMMITTED and REPEATABLE READ transaction isolation levels.

Controlling Transactions

The following describes transaction operations supported by the database:

  • Starting transactions

    You can start transactions using START TRANSACTION and BEGIN.

  • Setting transactions

    You can use the SET TRANSACTION or SET LOCAL TRANSACTION syntax to set transactions.

  • Committing transactions

    You can commit all operations of a transaction using COMMIT or END.

  • Rolling back transactions

    Rollback indicates that the system cancels all changes that a transaction has made to a database if the transaction fails to be executed due to a fault.

Transaction Isolation Levels

A transaction isolation level specifies how concurrent transactions process the same object.

img NOTE: The isolation level of a transaction cannot be reset after the first clause (SELECT, INSERT, DELETE, UPDATE, FETCH, COPY) for modifying data is executed in the transaction.

  • READ COMMITTED: At this level, a transaction can access only committed data. This is the default level.

    The SELECT statement accesses the snapshot of the database taken when the query begins. The SELECT statement can also access the data updates in its session, regardless of whether they have been committed. Note that different database snapshots may be available to two consecutive SELECT statements for the same transaction, because data may be committed for other transactions while the first SELECT statement is executed.

    At the READ COMMITTED level, the execution of each statement begins with a new snapshot, which contains all the transactions that have been committed by the execution time. Therefore, during a transaction, a statement can access the result of other committed transactions. Pay attention to whether a single statement always accesses absolutely consistent data in a database.

    Transaction isolation at this level meets the requirements of many applications, and is fast and easy to use. However, applications performing complicated queries and updates may require data that is more consistent than this level can provide.

  • REPEATABLE READ: A transaction can only read data committed before it starts. Uncommitted data or data committed in other concurrent transactions cannot be read. However, a query can read earlier data updates in its transaction, regardless of whether they have been committed. READ UNCOMMITTED differs from this level in that a transaction reads the snapshot taken at the start of the transaction, not at the beginning of the current query within the transaction. Therefore, the SELECT statement within a transaction always reads the same data, and cannot read data submitted by other concurrent transactions after the transaction starts. Applications at this level must be able to retry transactions, because serialization failures may occur.