Describe about major performance enhancements in oracle Database 12c.

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1. The optimizer will not use dynamic plan changes while executing called Adaptive Query Optimization.
2. If any object is missing the statistics or found stale during execution of plan, dynamic statistics (aka sampling) will be collected with new level 11.
3. Optimizer now stores the execution information in sysaux tablespace about the individual object execution statistics and create directives called SQL plan directives which can be used later execution to optimize further the statement 
4. Clustering factor can be set according to the index skewness, not leaving to Oracle 
5. CTAS now collect the statistics
6. Global Temporary tables have private session statistics no more it uses the one statistics for all, very useful for different batches run at same time 
7. DBMS_STATS now can show in report mode to tell how much does it take

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