DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

In addition, with V8, PQ89070 implements lock avoidance for a singleton SELECT statement
with ISOLATION(CS) and CURRENTDATA(YES). Semantically, a singleton select with
ISOLATION(CS) can avoid getting an S page or row lock regardless of the setting of the
CURRENTDATA parameter, since the cursor is closed after the SELECT is processed.
Lock avoidance for embedded SELECT or SELECT INTO
Semantically, a SELECT INTO with ISOLATION(CS) should avoid getting an S page or row
lock regardless of the setting of the CURRENTDATA(CD) option since the cursor is closed
after the SELECT is processed. Currently, with ISO(CS) and CD(NO) we get LOCK
AVOIDANCE but not with ISO(CS) and CD(YES).
If DB2 can determine that the data that it is reading has already been committed, it can avoid
taking the lock altogether. For rows that do not satisfy the search condition, this lock
avoidance is possible with CURRENTDATA(YES) or CURRENTDATA(NO). For a row that
satisfies the search condition on such a SELECT, lock avoidance is possible with
CURRENTDATA(YES) or CURRENTDATA(NO). For other rows that satisfy the search
condition, lock avoidance is possible only when you use the option CURRENTDATA(NO).
In a test case, executing a simple select statement in a loop (100k times), we have seen up to
a 25% better elapsed time in V8 when compared to the run without lock avoidance. When
compared to V7, V8 is 7% better with lock avoidance.
Chapter 4. DB2 subsystem performance 215