Discoverer Optimizer

Question

One of our application uses rule based optimizer at db level and by default discoverer uses cost based optimizer. We do a lot of ad-hoc reporting in this area. Does changing the optimizer at discoverer will help us in performance?

Answer 1

and one more ...
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Subject:Which Optimizer Is Being Used In Discoverer? : Cost or Rule
     Doc ID:      Note:1055786.6      Type:      PROBLEM
     Last Revision Date:      26-NOV-2002      Status:      PUBLISHED
PROBLEM DESCRIPTION:
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Does Discoverer use the cost-based or rule-based optimizer?
SOLUTION DESCRIPTION:
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There have been a number of questions recently about how the optimizer
chooses execution paths based on the presence of hints, statistics and the
optimizer_mode init.ora parameter. The rules are described in detail in
Chapter 8 of the Oracle 8 Tuning Guide.
There have also been questions about the Discoverer registry entry
"UseOptimizerHints".
If you do not need the details, for best results:
* leave "UseOptimizerHints" to its default value of 0,
* ensure the init.ora parameter optimizer_mode is set to CHOOSE,
* analyze the application tables, using ANALYZE TABLE command.
Which optimizer will be used:
The optimizer used will be the COST optimizer if there is a valid optimizer
hint in the SQL statement that Discoverer generates, regardless of all other
factors. Check the SQL inspector to look at the SQL generated. The kernel
parses SQL comments to determine if the comment is really a hint, and if so,
will ALWAYS, UNDER ALL CIRCUMSTANCES use the cost optimizer, even if there
are no statistics, and even if the init.ora parameters are set to use the
rule optimizer by default. Only if there is no optimizer hint in the SQL
statement, the following table shows what happens:
Optimizer Mode     Got           Optimizer used
parameter     Statistics?
CHOOSE          Y          Cost
CHOOSE          N          Rule
RULE          Y          Rule
RULE          N          Rule
ALL_ROWS     Y          Cost
ALL_ROWS     N          Cost (stats are estimated based on table
storage info)
FIRST_ROWS     Y          Cost
FIRST_ROWS     N          Cost (stats are estimated based on table
storage info)
In general, it makes sense to set the optimizer to CHOOSE, when the
presence of statistics (generated from the analyze table command) will
determine which optimizer is best. In Discoverer, we will not get Query
Performance prediction unless the COST optimizer is used, since the QPP
algorithm depends on getting an estimated cost for the query.
How do I get an optimizer hint in the query?
First, only consider this if you are familiar with Oracle tuning. You
don't need this for query prediction; you only do this if you want to
force a particular access path method on the optimizer. Set the hint in
the folder properties dialog. See the Oracle 8 Tuning Guide for details
of syntax.
NOTE: Any correct hint will force use of the cost optimizer under
ALL circumstances, even if the tables are not analyzed.
What does the registry entry "UseOptimizerHints" do?
The default value is 0 and it should not be changed. The registry entry
is only there to allow compatibility with Discoverer 3.0.7 which always
generated an optimizer hint (+All ROWS or +FIRST ROWS) in the SQL.  If 
you set "UseOptimizerHints = 1, it will revert to this behavior. The
effect of this is that the cost optimizer will be used under ALL
circumstances (see above). If your tables have not been analyzed, this
can result in poor query performance. UseOptimizerHints should always be
set to 0. If the tables are analyzed, you will use the cost optimizer,
and if they are not you will use the rule based optimizer.
What is the effect on Query Prediction?
Query prediction depends on Discoverer getting a cost for the query from
the cost optimizer. If you follow the guidelines above, and the tables are
analyzed, you will get query prediction, without having to worry about
optimizer hints. If the rule optimizer is used, due to one of the factors
above, you will not get query prediction. Setting "UseOptimizerHints = 1"
forces use of the cost optimizer, and you WILL then get query prediction,
but, unless the tables are analyzed you will get poor accuracy and poor
query performance, since the cost optimizer will not have the information
it needs to determine the best access path. And, of course, if the tables
ARE analyzed, and your optimizer_mode is set to CHOOSE, as recommended, you
will get query prediction and good performance, so you don't need the
UseOptimizerHints setting.
NOTE: To get query prediction, you need to ensure the steps outlined
in the release notes are also followed.
The behavior of the OPTIMIZER_MODE parameter is the same in Oracle7 and
OraCLE8, so this solution is correct for both.
REFERENCES:
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Discoverer Release Notes (as of 3.0.8)
Oracle Server Tuning Guide, Release 8.0.x