SQL/MX 2.x Reference Manual (H06.10+, J06.03+)

Introduction
HP NonStop SQL/MX Reference Manual—544517-008
1-7
Data Consistency and Access Options
Data Consistency and Access Options
Access options for DML statements affect the consistency of the data that your query
accesses.
For any DML statement, you specify access options by using the FOR option
ACCESS clause and, for a SELECT statement, by using this same clause, you can also
specify access options for individual tables referenced in the FROM clause.
The possible settings for option in a DML statement are:
The SQL/MX default access option for DML statements is READ COMMITTED.
However, you can set your system default for access options by specifying entries in
the SYSTEM_DEFAULTS table. See ISOLATION_LEVEL on page 10-47.
The implementation for REPEATABLE READ and SERIALIZABLE access options is
equivalent. This entry uses SERIALIZABLE for purposes of illustration.
For related information about transactions, see Transaction Isolation Levels
on
page 1-21.
SQL/MP Considerations
READ COMMITTED Specifies that the data accessed by the DML statement
must be from committed rows.
READ UNCOMMITTED
Specifies that the data accessed by the SELECT statement
need not be from committed rows.
SERIALIZABLE or
REPEATABLE READ
Specifies that the DML statement and any concurrent
process (accessing the same data) execute as if the
statement and the other process had run serially rather
than concurrently.
SKIP CONFLICT
Allows transactions to skip rows locked in a conflicting
mode by another transaction. SKIP CONFLICT cannot be
used in a SET TRANSACTION statement.
STABLE
Specifies that the row being accessed by the SELECT
statement is locked while it is processed, but concurrent
use of the database is allowed. STABLE is an ANSI
extension.
Note. If your SQL/MP application uses the BROWSE, STABLE, and REPEATABLE keywords,
NonStop SQL/MX accepts these keywords as synonyms for statement-level access options
READ UNCOMMITTED, STABLE, and SERIALIZABLE (or REPEATABLE READ), respectively.