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

Before ALTER INDEX add column, a table space scan was used to access the data. After
ALTER INDEX ADD of an existing column of the table to an index, the index was placed in a
RBDP state and needs to be rebuilt. After rebuild, the access path changed from table space
scan to non-matching index scan showing a 10 times elapsed time improvement.
Class 1 Time (seconds)
120
100
Figure 5-10 SELECT elapsed and CPU time before and after ALTER INDEX add column
Conclusion
A change of the access path from table space scan to an access path using the index
improves the elapsed time. This is well known. What is worth noting is the ease of
implementing the change. Remember that an index in RBDP state does not block access to
the data. For non-unique indexes, static plans and packages that do not use the index to
retrieve data, they will continue to function as before. For unique indexes placed in RBDP,
those plans and packages that are dependent on them will get a -904 (unavailable resource)
on an update of an existing row or insert of a new row. Deletes are no problem. For the time
until rebuild, the optimizer simply ignores the index for dynamic SQL; for static SQL a rebind is
necessary.
Recommendations
Once the index is available again, revert to optimal access paths by:
► Rebinding affected plans and packages for static SQL.
► Automatic invalidation of related statements in the dynamic statement cache when RBDP
is reset. At next execution, the statement is prepared again, and can go back to the
original access path.
Important: The enhancement is designed to allow maximum availability for the situations
where a new column is added to a table, and this new column is also desired as part of an
existing index. By making changes within the same unit of work there is no unavailability.
236 DB2 UDB for z/OS Version 8 Performance Topics
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SELECT elapsed time and CPU time
Elapsed Time CPU Time
Before Alter After Rebuild
Index
Index
(2) (5)
1. DDL Create Index
CREATE INDEX ix1 ON tab1(c1) ...
2. SELECT c1, c2 … WHERE c2 = ‘xx‘
Column not indexed, table space
scan
3. ALTER INDEX ix1 ADD COLUMN (c2)
Index in RBDP
4. Rebuild Index
5. SELECT c1, c2 … WHERE c2 = ‘xx‘
Non matching index only scan
10+ times Elapsed time
improvement