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

manage the number of qualifying rows and columns whenever possible. You should write
SQL statements that limit the range of rows that are read into the temporary table.
Although from a performance point of view dynamic scrollable cursors always outperform the
static cursor model, we do not recommend that the dynamic cursor model should always be
used over the static cursor model. As mentioned previously, some customers appreciate the
concept of optimistic locking (static cursor model only) since it contributes to concurrency.
The static cursor model should be considered if you have a requirement for an application
that must scroll back to a prior screen and retain and display the same values as before. Also,
for gathering certain reports, such as average outstanding balance in a certain zip code,
where the application programmer is not concerned about the minute-by-minute updates to
the outstanding balance or the newly inserted or deleted accounts, the static cursor model
with fetch insensitive can build these rows in a temporary table and permit the application to
manipulate the rows as necessary, allowing concurrency to the base table.
Besides performance, dynamic scrollable cursors are preferred when it is important for the
application to see/access updated as well as newly inserted rows. Maximum concurrency can
be achieved with isolation cursor stability.
The summary of the recommendations for static and dynamic cursors is:
► Static scrollable cursors can be the better solution if:
– You need to scroll backwards to obtain initial values
– You do not care about constant changes (such as business intelligence or
representative samples).
► Dynamic scrollable cursors can be preferable in situations where:
– It is important for the application to access/see updated/inserted rows
– Performance is important
► Use multi-row operations whenever possible, particularly for the dynamic cursor model.
3.13 Backward index scan
With the enhancements introduced to support dynamic scrollable cursors, DB2 also provides
the capability for backward index scans. This allows DB2 to avoid a sort and it can eliminate
the need for an index. With this enhancement, available in CM, it is no longer necessary to
create both an ascending and descending index on the same table columns.
To be able to use the backward index scan, you must have an index on the same columns as
the ORDER BY and the ordering must be exactly opposite of what is requested in the
ORDER BY.
For example, if you have created the index
CREATE UNIQUE INDEX ON (A DESC, B ASC)
then DB2 V8 can do a forward index scan for a
SELECT ... FROM ... ORDER BY A DESC, B ASC
and a backward index scan for
SELECT ... FROM ... ORDER BY A ASC, B DESC
Chapter 3. SQL performance 107