SQL/MX 2.x Reference Manual (H06.04+)

Introduction

HP NonStop SQL/MX Reference Manual—540440-003

1-21

Transaction Access Modes

A transaction has an access mode that is either READ ONLY or READ WRITE. You

can set the access mode of a transaction by using a SET TRANSACTION statement.

See SET TRANSACTION Statement on page 2-218.

READ ONLY

If a transaction is executing with the READ ONLY access mode, statements within the

transaction can read, but cannot insert, delete, or update, data in tables. This

restriction means that among the DML statements, only the SELECT statement can

execute within that transaction.

If the transaction isolation level is READ UNCOMMITTED, the default access mode is

READ ONLY. Further, for READ UNCOMMITTED, you can specify only READ ONLY

explicitly by using the SET TRANSACTION statement.

READ WRITE

If a transaction is executing with the READ WRITE access mode, statements within the

transaction can read, insert, delete, or update data in tables. Therefore, any DML

statement can execute within that transaction.

If the transaction isolation level is not READ UNCOMMITTED, the default access

mode is READ WRITE. However, you can specify READ ONLY explicitly by using the

SET TRANSACTION statement.

Transaction Isolation Levels

A transaction has an isolation level that is either READ UNCOMMITTED, READ

COMMITTED, or SERIALIZABLE or REPEATABLE READ. The SQL/MX

implementation for REPEATABLE READ and SERIALIZABLE is equivalent.

SERIALIZABLE is used for purposes of illustration.

You can set the isolation level of a transaction explicitly by using a SET

TRANSACTION statement. See SET TRANSACTION Statement on page 2-218.

You can set your system default for the transaction isolation level by specifying the

ISOLATION_LEVEL entry in the SYSTEM_DEFAULTS table. The default isolation level

of a transaction is determined according to the rules specified in ISOLATION_LEVEL

on page 10-45.

READ UNCOMMITTED

This isolation level allows your transaction to access locked data. You cannot use

READ UNCOMMITTED for transactions that modify the database.

READ UNCOMMITTED provides the lowest level of data consistency. A transaction

executing with this isolation level is allowed to: