A live database is a constant moving target and we need to keep an open mind about changes which happen all the time,
such as data changes, data stats changes, database parameter changes, database patch or upgrade, etc.
Oracle’s CBO engine automatically evolves SQL plan as a mechanism to respond to those changes,
which could lead to SQL plan deterioration and performance issue.
SQL plan change will cause database performance issue if the new plan requires more resources and/or more time
for running the same SQL statement and processing the same amount of data. The time model analysis and resource
consumption analysis need to be applied to both the old plan and new plan, in order to identify if SQL plan change
is the cause of performance issue.
Detecting SQL plan change serves as the start of investigation, while
ensuring an optimal SQL plan marks as the happy ending of SQL tuning.