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Facilidade de uso
Optimizer and Executor
Ways to Influence the Query Execution Plan
Ways to Influence the Query Execution Plan
In NonStop SQL/MP, you can write queries in different ways to affect how a query is
optimized. You must know how the SQL/MP optimizer handles predicates so that a
proper query design yields an optimal plan. You can write queries in such a way as to
eliminate certain types of joins. Query performance in NonStop SQL/MX is not affected
by query design, because the SQL compiler performs the majority of transformations
internally.
This list indicates a few actions you can take to influence a query execution plan in
NonStop SQL/MX:
• Setting default values. See Default Settings.
• Updating statistics regularly.
• Forcing query execution plans with the CONTROL QUERY SHAPE statement.
For detailed information about improving query performance, setting default values,
updating statistics, and forcing query execution plans, see the SQL/MX Query Guide.
Default Settings
In NonStop SQL/MP, you specify certain default settings through the CONTROL
directives. For example, the CONTROL EXECUTOR directive allows you to specify
whether you want parallel processing on or off. With the CONTROL TABLE directive,
you can specify that you want to use a certain join method or change the timeout
setting. With the CONTROL QUERY directive, you can adjust other default values.
In NonStop SQL/MX, the SYSTEM_DEFAULTS table is a user table that enables you
to specify default settings for options and other attributes when executing MXCI
commands and SQL queries that can be run through MXCI or within an application.
For some attributes, you can override a default value by specifying an option with the
statement or command. You might find it more efficient, however, to configure the
SQL/MX execution environment with default values. NonStop SQL/MX lets you change
the default settings of many more items than NonStop SQL/MP.
If you do not enter an option when entering a command or if you do not provide some
attribute associated with the execution of queries, NonStop SQL/MX chooses a default.
If you have specified system default settings for the execution environment, these
value are used. Otherwise, the SQL/MX default values are used.
In addition to configuring the execution environment with default values through the
defaults tables, in NonStop SQL/MX you can use the CONTROL TABLE and
CONTROL QUERY DEFAULT settings to change default values at run time. For more
information about these commands, see Section 3, DML Features and the SQL/MX
Reference Manual.
NonStop SQL/MX stores system defaults information in the SQL/MP user table named
DEFAULTS, which is registered in the system catalog. For a full listing of default values
that can be changed, see the SYSTEM_DEFAULTS Table entry in the SQL/MX
Reference Manual. See also the SQL/MX Query Guide.
HP NonStop SQL/MX Comparison Guide for SQL/MP Users— 523735-003
5 -7
... Este manual também é adequado para os modelos :
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