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

Query performance problem
Lack of multi-column statistics can cause a bad access path and poor query performance
when a table is referenced by two or more equal, range or BETWEEN predicates.
The possibility of hitting this problem increases when:
► The number of predicates referencing one table increases
► The number of joined tables in a query increases
Two, often complementary, DB2 V8 functions help with this issue:
► Collecting additional statistics
► Relying on DB2 V8 better filter factor estimation
Additional statistics
Collecting cardinality and multi-column distribution statistics and rebinding is the first step to
solve the problem. The Statistics Advisor, of Visual Explain V8, can generate automatically
the RUNSTATS statement for the query, after using the Analyze function for a multi-predicate
SQL text. See 6.4, “RUNSTATS enhancements” on page 272. Visual Explain is discussed in
3.15, “Visual Explain” on page 110 and 3.15.3, “Statistics Advisor” on page 116.
Example 3-6 shows one SQL statement text used as input to Visual Explain. This is the old
query 5 used to verify the DB2 Catalog consistency, discussed in Chapter 9, “Installation and
migration” on page 341.
Example 3-6 SQL text for Visual Explain
SELECT DBNAME, NAME FROM SYSIBM.SYSTABLESPACE TS
WHERE NTABLES
(SELECT COUNT(*) FROM SYSIBM.SYSTABLES TB
WHERE TYPE IN ('T', 'X','M')
AND TB.DBNAME = TS.DBNAME
AND TB.TSNAME = TS.NAME
AND TS.NAME 'SYSDEFLT');
The query has one correlated subquery.
The outer query scans the SYSIBM.SYSTABLESPACE table which has the indicated number
of tables NTABLES different from the counts of tables in the inner query based on table
SYSIBM.SYSTABLES. For each qualified row in the outer table, the inner query is
re-executed. Table type = “X” specifies auxiliary table at the current server for storing LOB
data.
The Explain function in Visual Explain for the SQL text results in the graphic for the access
path selected by DB2 V8 in Figure 3-29.
72 DB2 UDB for z/OS Version 8 Performance Topics