Oracle Magazine, July/August 2019
Database Developer and DBA ETL optimizer modes National Language Support NLS settings optimizer parameter ORACLE MAGAZINE JULY AUGUST 2019 139 settings and so on may be different The data types of any bind variables can influence the execution plan See The Importance of Data Types sidebar for an example The values within bind variables can be examined at execution time and influence optimizer decisions but this examination is not done during an EXPLAIN PLAN command As the optimizer becomes more powerful with each release of Oracle Database many more scenarios where EXPLAIN PLAN does not get the true execution plan are possible such as when previous executions of the same SQL statement are remembered by the database and help determine the best execution plan for future executions A full treatment of these enhancements is beyond the scope of this SQL tuning series but the core message here is Do not rely on the EXPLAIN PLAN command result to be an indicator of the true execution plan This may seem like a contradiction but this is only because historically many Oracle professionals have used the terms execution plan and explain plan interchangeably They are not actually the same though and as the Oracle Database optimizer continues to be enhanced and improved there will be more and more differences between the two To make the difference between the result of an EXPLAIN PLAN command and the actual execution plan clear the execution plan is the plan that was used at runtime during the execution of the SQL statement Having this execution plan is critical to
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