Skipping Locked Rows [DB2 9 for z/OS]

In DB2 9 it is possible for a transaction to skip over rows that are locked. This can be accomplished by means of the SKIP LOCKED DATA option within your SQL statement(s). SKIP LOCKED DATA can be specified in SELECT, SELECT INTO, and PREPARE, as well as searched UPDATE and DELETE statements. You can also use the SKIP LOCKED DATA option with the UNLOAD utility.

Of course, if a program skips over locked data then that data is not accessed and the program will not have it available. When this option is used DB2 will just skip over any locked data instead of waiting for it to be unlocked. The benefit, of course, is improved performance because you will not incur any lock wait time. But it comes at the cost of not accessing the locked data at all. This means that you should only utilize this clause when your program can tolerate skipping over some data.

The SKIP LOCKED DATA option is compatible with cursor stability (CS) isolation and read stability (RS) isolation. But it cannot be used with uncommitted read (UR) or repeatable read (RR) isolation levels. DB2 will simply ignore the SKIP LOCKED DATA clause under UR and RR isolation levels.

Additionally, SKIP LOCKED DATA works only with row locks and page locks. That means that SKIP LOCKED DATA does not apply to table, partition, LOB, XML, or table space locks.
Let's look at an example. Suppose we have a table with 5 rows in it that looks like this:

KEY  FNAME  LNAME
---  ------ -------
 1   JOE     MAMA
 2   DON     KNOTTS
 3   KIM     PORTANT
 4   BOB     NOBBLE
 5   KIM     BIMBO

Assume row level locking. Next assume that an UPDATE statement is run against the table changing FNAME to JIM WHERE LNAME = 'KIM'. And it is hanging out there without a COMMIT. Next, we run:

SELECT COUNT (*)
FROM   TABLE
WHERE  FNAME >= ’AAA’
SKIP LOCKED DATA;

The count returned would be 3 because DB2 skips the two locked rows (rows 3 and 5). And, of course, if the locks are released the count would be 5 again.