Reasons Of Changing Execution Plans ?
* Stale or missing statistics
* Have gathered fresh statistics
* Optimizer engine has changed after a database upgrade
* Index has been added or dropped or Table structure has changed
* Database optimizer related init.ora parameters have been changed
* Parsed representation of the SQL statement is not in the Library Cache – statements are aged out
* Reparsing the same SQL statement now leads to generation of a new plan
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