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

Table 5-5 DPSI overhead
Access path PLC - index only
EVE - index only
The DPSI overhead is explained by DB2 having to probe each DPSI partition in order to
evaluate the predicates. The access path remains the same in both test cases.
In order to illustrate an access path change from changing use of a NPI to a DPSI we
evaluated the following query:
SELECT COUNT(DISTINCT CSCD) FROM CVR
Table 5-6 shows the measurement results.
Table 5-6 DPSI access path change
NPI DPSI % difference
PLC - index only
EVE - index only
CPU (sec.) 0.021699 0.083861 + 281
Elapsed (sec.) 0.037188 0.305458 + 721
Index getpages 8 17,000
NPI DPSI % difference
Access path index only table space scan + sort
CPU (sec.) 1.21221 8.30175 + 585
Elapsed (sec.) 1.317186 24.67209 + 1773
Getpages 2k index getpages 139k data getpages +
29k work file getpages
Conclusion
Utility processing benefits from the use of DPSIs. Query processing using only the DPSI in
the access path has to probe every partition of the DPSI to evaluate the predicates due to the
non-unique nature of the DPSI. In a data sharing environment, if some workloads are partition
and member specific, reduced data sharing overhead is possible with DPSIs as intersystem
read/write interest on physical partitions can be eliminated vs. non-partitioned indexes, where
keys belonging to different data partitions are spread throughout the non-partitioned index.
Recommendations
Replacing all NPIs by DPSIs is not the objective. NPIs will still prove to be useful. Utilities will
definitely benefit from DPSIs while contention at the logical partition level is eliminated.
Queries that combine predicates on the partitioning columns and the DPSI will perform well.
Explicitly coding the partitioning columns in the predicate allows the DB2 optimizer to
eliminate not qualifying partitions from the query. This is called partition pruning. Pruning can
be deferred until execution time if host variables are used. The pruning takes place once the
contents of the host variables are known.
When using DPSIs part of the responsibility is shifted from DBA to development. Coding the
correct predicates to allow for partition pruning is crucial to performance.
The use of DPSIs versus NPIs is dictated by availability considerations. If, for example, even
the outage due to the BUILD2 phase of an online REORG cannot be tolerated DPSIs are the
solution.
Chapter 5. Availability and capacity enhancements 255