Best Practices for SAP BI using DB2 9 for z/OS - IBM Redbooks

Figure 10-14 Comparing filtering
The counts in Figure 10-14 demonstrate that filtering of local predicates on the
D6 table (37 rows), and combined D6 to F table (1,317,164 rows).
This step is repeated for each set of dimension/snowflakes independently and
joined with the facttable. These counts will be used to compare all filtering
combinations.
Step 4 - Compare Filtering
Join Count with Table CARDF % Rows
Sequence Filtering
Returned
Z1
Z1-X2
Z1-X2-D1
Z1-X2-D1-F
X1
X1-D3
X1-D3-F
DP
DP-F
D5
D5-F
13
26,514
171,787
1,702,101
258
281
511,179
432
10,743,216
598
10,743,216
13
353,413
1,565,004
10,743,216
43,511
30,932
10,743,216
432
10,743,216
598
10,743,216
100%
} 7.50%
10.98%
15.84%
0.59%
0.26%
4.76% }
100%
100% }}
100%
100%
D6 37 423 8.75%
D6-F 1,317,164 10,743,216 12.26%
The original facttable contains 10,743,216 rows.
Comparing the filtering provided by each dimension/snowflake joined to the fact,
we can see in Figure 10-14 that neither DP nor D5 provide any filtering (because
the count of join to the fact equals the count of the fact only). The snowflake
containing D3 provides the strongest filtering, followed by the dimension D6, and
the snowflake containing D1.
Therefore, if the current access path does not involve the strongest filtering
dimension (D3, accessed before the facttable), then the access path may be
suboptimal.
If the access path does not reflect the filtering as outlined by the counts, then the
first step is to analyze the statistics and ensure that RUNSTATS was run. (This
may seem like an obvious suggestion, but it is surprising how often RUNSTATS
218 Best Practices for SAP BI using DB2 9 for z/OS
}
#3
#1
No Filtering
No Filtering
#2