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

3.4.4 Conclusions
Summary of measurements with a predicate comparing a string data type with different
lengths:
► Elapsed time in V7 = 10.65 sec. and in V8 = 1.11 sec. (reduction of 89%)
► CPU time in V7 = 3.24 sec. and in V8 = 0.08 sec. (reduction of 97%)
► Predicates comparing mismatched string data types are stage 1 and indexable with minor
exceptions.
► Predicates comparing mismatched numeric data types are stage 1 and indexable with
minor exceptions.
3.4.5 Recommendations
► Consider rebinding plans or packages to get better access paths.
► Consider creating an index for the mismatched columns to enable index access.
3.4.6 Considerations on CASTing
In DB2 V7, we recommend you use the CAST function to make both data types the same.
However, if you do this, first you add some overhead, and then you generally dictate the join
sequence because DB2 favors the index and picks the simple column side as inner table. Let
us look at the following SELECT:
SELECT *
FROM CUST AS C, ORD_DEC AS O
WHERE C.CUST_NO = O.CUST_NO;
If you rewrite the predicate as ‘.CUST_NO = CAST(O.CUST_NO AS INTEGER)’, DB2 very
likely picks the CUST table as inner table. Furthermore, since ORD_DEC.CUST_NO is
DEC(15,0), this has a bigger range than the INTEGER type. Unless you are very sure that all
the values in the ORD_DEC.CUST_NO are within the range of INTEGER, the CAST may fail
and DB2 may return an error. In DB2 V8, unless you really want to dictate the join sequence,
it is better not to add any CAST and let DB2 choose the join sequence based on costing.
Generally this provides better performance.
3.5 Multi-predicate access path enhancement
Queries which contain two or more predicates referencing a table can get a better access
path with statistics for a group of columns (non-indexed or non-leading index columns)
collected by RUNSTATS in V8.
Long running queries with skewed data distribution or column correlation type environment
involving joins with multiple predicates on columns that are not the leading columns of an
index or columns that do not participate in indexes are good candidates to benefit from
improved access paths.
The new access path in V8 can change the sequence of tables participating in joins involving
multiple tables to take advantage of early filtering, and as a result, accessing fewer pages to
execute the SQL statement.
Chapter 3. SQL performance 71