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

Introduction
HP NonStop SQL/MX Reference Manual—523725-004
1-7
SQL/MP Considerations
The implementation for REPEATABLE READ and SERIALIZABLE access options is
equivalent. This entry uses SERIALIZABLE for purposes of illustration.
For information about transactions, see Transaction Isolation Levels on page 1-20.
SQL/MP Considerations
READ UNCOMMITTED
This option enables you to access locked data. READ UNCOMMITTED is not available
for DML statements that modify the database. It is available only for a SELECT
statement.
READ UNCOMMITTED provides the lowest level of data consistency. A SELECT
statement executing with this access option is allowed to:
•
Read data modified by a concurrent process (sometimes referred to as dirty reads)
•
Read different committed values for the same item at different times or find that the
item no longer exists (sometimes referred to as nonrepeatable reads)
•
Read different sets of committed values satisfying the same predicate at different
times (sometimes referred to as phantoms)
READ COMMITTED
This option allows you to access only committed data.
The implementation requires that a lock can be acquired on the data requested by the
DML statement—but does not actually lock the data, thereby reducing lock request
conflicts. If a lock cannot be granted (implying that the row contains uncommitted
data), the DML statement request waits until the lock in place is released.
READ COMMITTED provides the next higher level of data consistency (compared to
READ UNCOMMITTED). A statement executing with this access option does not allow
dirty reads, but both nonrepeatable reads and phantoms are possible.
READ COMMITTED provides sufficient consistency for any process that does not
require a repeatable read capability.
SERIALIZABLE or REPEATABLE READ
This option locks all data accessed through the DML statement and holds the locks on
data in audited tables until the end of any containing transaction.
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.