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

3.6.2 Conclusion
Figure 3-33 Query parallelism in V8
Queries involving joins using MCSMJ (the method is likely to be used when the result set
tends to be large) can have benefits because of access path enhancements due to better
filtering when multiple columns are used as predicates.
Enabling query parallelism in long running queries can give extra improvement in elapsed
time, but generally an increase in CPU time.
3.6.3 Recommendations
Query parallelism in V8
SELECT COUNT(*) FROM P,PX
WHERE P.KEY < 1000
AND P.BD_C20=PX.BD_VC20
AND P.CT_C10 = PX.CT_VC10;
Data type of P.BD is CHAR(20)
Data type of P.CT is CHAR(10)
V8 PLAN_TABLE
DEGREE(ANY)
TBNAME METHOD MATCHCOL
Evaluate the usage of sort merge join in long running queries. V8 allows improvement if there
are multi-column predicates in sort merge join. If the elapsed time is a concern, the query can
be executed in shorter elapsed time if executed with query parallelism.
3.7 Parallel sort enhancement
PX
P
V8 PLAN_TABLE
(continue)
ACCESSTYE
R
I
0
2
ACCESS
DEGREE
30
?
In V7, parallel sort has the following characteristics:
► Parallel sort enabled for single-table sort composite only
► Not cost-based decision
► No threshold to disable parallel sort for short running queries
0
1
JOIN
DEGREE
?
3
Elapsed sec CPU sec
Sort composite tables from more than one table can have longer elapsed time because sort is
not executed in parallel. If the composite table is from a single table, then parallel sort is
invoked in V7.
80
70
60
50
40
30
20
10
0
V8 DEGREE(1)
V8 DEGREE(ANY)
Chapter 3. SQL performance 79