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

Table 6-4 RUNSTATS with DSTATS measurement data in V8
Class 1
elapsed
(sec.)
Class 1
CPU time
(sec.)
DSTATS
CL1 ELT
(sec.)
DSTATS
CL1 CPU
(sec.)
DSTATS
Overhead
ELT (%)
DSTATS
Overhead
CPU (%)
Case 1
RUNSTAT
S
table
Case 2
RUNSTAT
S
table idx
Case 3
RUNSTAT
S
table
1 column
Case 4
RUNSTAT
S
table
1 colgrp
w/ 2 cols
Case 5
RUNSTAT
S
table idx
1 colgrp
w/ 2 cols
26.04 29.26 28.00 27.99 30.59 39.67
19.10 21.62 24.50 24.32 26.62 32.22
N/A N/A 1.96 1.94 1.32 13.63
N/A N/A 5.39 5.22 5.00 13.12
N/A N/A 7.5 7.5 4.5 52.3
N/A N/A 28.3 27.3 23.1 68.7
In V8 we can observe the CPU overhead when running RUNSTATS specifying distribution
statistics. In case 3, which is the base scenario of DSTATS, we see a 28.3% overhead in CPU
time, and in case 4 (adding 1 column to the COLGROUP) we see 27.3%.
However, in Case 6 (2 column groups) it is 68.7%. We can conclude that, in V8, increasing
the number of column groups drives the CPU cost higher than increasing the number of
columns.
There is almost no difference when 1 column is added in 1 column group (Case 3 to 4). On
the other hand, there is 29.7% degradation in elapsed time and 21% degradation in CPU time
of the total RUNSTATS time, when the column group of statistics goes to 2 (Case 4 to 6).
In general, V8 RUNSTATS cuts down the elapsed time well compared to V7.
Case 6
Runstats
table
2 colgrp
w/ 2 ,1 cols
Access path improvement
To verify the usefulness of DSTATS we have tested the access path improvement of 6 queries
out of 25 queries of the query workload after running distribution statistics. We have gathered
distribution statistics on 10 columns manually picked. After that, most of the queries have
shown positive access path changes.
Bind cost
You need to consider that RUNSTATS executions, by adding the frequency values in the
SYSIBM.SYSCOLDIST catalog table, cause high bind cost. Especially high CPU
consumption occurs for dynamic prepare when FREQVAL COUNT in SYSCOLDIST is
greater than 100. In this case a large portion of CPU time is used to remove duplicate
frequency values. PTF UK00296 for APAR PQ94147 is meant to reduce such large bind cost.
Chapter 6. Utilities 279