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

Percent of index pages
Avg. Index Leaf Pages and Getpages for 15 Queries
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-20
-30
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-50
Figure 5-14 Average index leaf pages and index getpages
Initially there is no benefit from not padded index keys as together with the key also the length
bytes are saved. The longer the VARCHAR becomes the more the benefit from not padding
becomes visible. Index storage requirements could be reduced as only the actual data is
stored and the index tree may have fewer levels.
In a third measurement the impact of NOT PADDED indexes on utilities was measured. We
used a table with 10 M rows and one PADDED or NOT PADDED index. The following cases
were measured:
1. 10 M rows table with a one-column indexed key of VARCHAR(14) with varying length from
1 to 14.
2. 10 M rows table with a one-column indexed key of VARCHAR(1)
3. 10 M rows table with a four-column indexed key of each VARCHAR(1)
The tests were run for LOAD, REORG table space, REORG index, REBUILD, CHECK index
and RUNSTATS index.
See Figure 5-15 for the measurement result of test case 1.
240 DB2 UDB for z/OS Version 8 Performance Topics
20
10
0
Queries using NOT PADDED vs. PADDED keys
VC10 VC20 VC30 VC40 VC50
# of Leaf Pages
Index Getpages