Neoview Control Query Default (CQD) Reference Guide (R2.5)

This setting is to be used only for forcing a desired join order that was not generated by
default by the optimizer. It can be used as a workaround for query plans with inefficient
join order.
Production usage
Because you are in effect forcing the optimizer to use a plan that joins the table in the order
specified in the FROM clause, the plan generated may not be the optimal one.
Impact
QueryLevel
Not applicableConflicts/Synergies
Not applicableAddressing the real
problem
Neoview Release 1.0Introduced in release
Not applicableDeprecated in release
MDAM_SCAN_METHOD
Influencing Query PlansCategory
Enables or disables the Multi-Dimensional Access Method.Description
MDAM is considered.‘ON’
MDAM is disabled.‘OFF’
The default value is ‘ON’.
Values
In certain situations, the optimizer might choose MDAM inappropriately, causing poor
performance. In such situations you may want to turn MDAM OFF for the query it is
effecting.
Usage
Not applicableProduction usage
Table scans with predicates on non-leading clustering key column(s) could benefit from
MDAM access method if the leading column(s) has a small number of distinct values.
Turning MDAM off results in a longer scan time for such queries.
Impact
Set this CQD at the query level when MDAM is not working efficiently for a specific query.
However, there may be cases (usually a defect) where a larger set of queries is being
negatively impacted by MDAM. In those cases you may want to set it at the service or
system level.
Level
Not applicableConflicts/Synergies
Not applicableAddressing the real
problem
Neoview Release 1.0Introduced in release
Not applicableDeprecated in release
MERGE_JOINS
Influencing Query PlansCategory
Determines if Merge Join is considered by the optimizer to generate an execution plan.Description
Merge Join is considered.‘ON’
Merge Join is disabled.‘OFF’
The default value is ‘ON’.
Values
Use this CQD when you want to force a query plan not to use Merge Joins. This is useful
as a workaround for query plans with very expensive sorts for Merge Joins. Turning Merge
Join OFF also has the advantage of reducing the query compile time.
Usage
22 Optimizer