SQL/MX 2.x Reference Manual (G06.24+, H06.03+)

Introduction
HP NonStop SQL/MX Reference Manual—523725-004
1-14
Concurrency
If this option is set to ON, NonStop SQL/MX automatically commits any changes, or
rolls back any changes, made to the database at the end of statement execution.
AUTOCOMMIT is set ON by default at the start of an MXCI session.
If this option is set to OFF, the current transaction remains active until the end of the
MXCI session unless you explicitly commit or roll back the transaction. The default is
OFF for embedded SQL in a C or COBOL program. The default is ON for embedded
SQL in a Java program.
Concurrency
Concurrency is defined by two or more processes accessing the same data at the
same time. The degree of concurrency available—whether a process that requests
access to data that is already being accessed is given access or placed in a wait
queue—depends on the purpose of the access mode (read or update) and the
isolation level.
NonStop SQL/MX provides concurrent database access for most operations and
controls database access through the mechanism for locking and the mechanism for
opening and closing tables. For DML operations, access and locking options affect the
degree of concurrency. See Data Consistency and Access Options on page 1-6,
Database Integrity and Locking on page 1-9, and SET TRANSACTION Statement on
page 2-223.
This tables describe interactions between SQL/MX operations.
Table 1-1 compares operations with access modes and lists DDL and Utility operations
you can start while DML operations are in progress.
Table 1-1. Concurrent DDL/Utility Operation and File Access Modes
Access Mode
DDL Operations
You Can Start
READ
UNCOMMITTED
READ
COMMITTED STABLE SERIALIZABLE
ALTER INDEX Allowed Allowed Allowed Allowed
ALTER TABLE
attributes
Allowed
* Allowed* Waits Waits
* DDL operation aborts the DML operation