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

RUNSTATS INDEX PARTITION of a DPSI showed equivalent performance to a partition of a
PI. Comparing to RUNSTATS INDEX of a logical part of a NPI, the performance was better.
Performance measurement result for queries and DPSIs
We executed queries against 3 partitioned tables with an NPI and a DPSI on the same
columns. The measurements were done with and without parallelism.
The tables used are:
► CVR - 1,654,700 rows, 10 partitions
► EVE - 931,174 rows, 6 partitions
► PLC - 624,473 rows, 8 partitions
Attention: Make sure PTF UQ93972 for APAR PQ92227, which addresses SQL
performance problems with DPSIs, is applied.
Table 5-3 shows the measurement results for the following query:
SELECT COUNT(*) FROM CVR
Table 5-3 NPI vs. DPSI SELECT COUNT query
This is the case of an index scan. Almost the same number of index getpages are executed
for DPSI as for NPI. The results shown are those of the second execution of the queries (data
sets are all open) but the data is not preloaded. The query with predicates that only reference
DPSI indexed columns experiences a reduction in CPU time of 23% compared to an NPI on
the same columns. Since these queries are close to CPU bound, a similar reduction in
elapsed time of 19% is experienced.
The same query was also executed with parallelism enabled. The results are presented in
Table 5-4.
Table 5-4 NPI vs. DPSI SELECT COUNT query parallelism
Parallelism helps quite a lot in improving the elapsed time because of the probing in parallel of
the DPSI. The reduction in CPU is somewhat offset by the activation of parallelism, but the
elapsed time improvement is 64%.
Now we evaluate a query which qualifies data on NPI and DPSI columns in order to verify the
overhead of DPSI over NPI:
SELECT COUNT(*) FROM EVE, PLC WHERE EVE_DPSI>=PLC_DPSI AND PLC_DPSI = constant
Table 5-5 shows the results for this query.
254 DB2 UDB for z/OS Version 8 Performance Topics
NPI DPSI % difference
Access path index only (2285 pages) index only (2302 pages) =
CPU (sec.) 0.759703 0.587190 -23%
Elapsed (sec.) 0.825141 0.668409 -19%
NPI DPSI % difference
Access path index only (2312 pages) index only (2342 pages) =
CPU (sec.) 0.718432 0.657405 -9%
Elapsed (sec.) 0.637515 0.231345 -64%