Getting Started with DB2 Express-C

Chapter 13 – Concurrency and Locking 225
Statement isolation level can be set using the WITH {isolation level} clause. For example:
SELECT ... WITH {UR | CS | RS | RR}
Example scenario:
An application needs to get a "rough" count of how many rows are in a table. Performance
is of utmost importance. Cursor stability isolation level is required with the exception of one
SQL statement:
SELECT COUNT(*) FROM tab1 WITH UR
For embedded SQL, the level is set at bind time, for dynamic SQL, the level is set at run
time.
Choosing which isolation level to use depends on your application. If your application does
not need exact counts as in the above example, choose UR isolation. If your application
requires very tight control on the data it works with, choose RR isolation.
To use currently committed semantics at bind time or prepare time, use this syntax:
BIND:
>--+-------------------------------------------------------------+-->
'--CONCURRENTACCESSRESOLUTION--+--USE CURRENTLY COMMITTED--+--'
'--WAIT FOR OUTCOME---------'
PREPARE:
concurrent-access-resolution:
|-+-USE CURRENTLY COMMITTED-+--------------------------|
'-WAIT FOR OUTCOME--------'
On a JDBC application using the IBM Data Server Driver for JDBC and SQLJ, you can use
the concurrentAccessResolution property to enable currently committed.
13.5 Lock escalation
Every lock made by DB2 consumes some memory. When the optimizer thinks it is better to
have one lock on the entire table, rather than multiple row locks, lock escalation occurs.
Figure 13.9 illustrates this.