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

cases is the total time of RUNSTATS plus DSTATS. The overhead of DSTATS is derived from
the percentage of DSTATS time over RUNSTATS time (DSTATS time / RUNSTATS time *100).
Table 6-3 RUNSTATS with DSTATS measurement data in V7
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
Runstats
table
Case 2
RUNSTAT
S
table, idx
Take a look at the DSTATS overhead in Case 3 (collecting distribution statistics on 1 column):
DSTATS overhead is 20% for elapsed time. In V7 the overhead of using the DSTATS tool
shows a high percentage in general, because of the cost of running and binding DSTATS as a
standalone program on top of the cost of RUNSTATS. The largest overhead appears when
going from 1 column to 2 in one column group (case 3 to 4). In this case there is 24.8%
degradation in elapsed time and 31.3% degradation in CPU time.
When the number of column groups goes from 1 to 2 (case 4 to 6), there is 11% performance
degradation in elapsed time and 6.8% degradation in CPU time.
Table 6-4 shows the results of the same test cases with V8 RUNSTATS with distribution
statistics. DSTATS times are now the difference between RUNSTATS with distribution
statistics and RUNSTATS only time (for example, DSTATS in Case 3 is the difference of Case
3 and Case 1 in terms of class 1 time).
278 DB2 UDB for z/OS Version 8 Performance Topics
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
25.91 29.70 31.09 38.80 42.59 43.08
19.31 20.94 21.97 28.85 30.49 30.80
N/A N/A 5.18 12.88 12.88 17.17
N/A N/A 2.66 9.54 9.54 11.49
N/A N/A 20 49.7 43.4 66.3
N/A N/A 13.8 49.4 45.6 59.5
Case 6
RUNSTAT
S
table+
2 colgrp
w/ 2 ,1 cols