12c New Features to "Watch Out For"

I think that these 12c New Features are such that their behaviour may create surprises.  Be ready to catch unexpected behaviour.

1.2.4.1 Adaptive Query Optimization

1.2.4.7 Dynamic Statistics

1.2.4.11 Improved Automatic Degree of Parallelism

1.3.3.1 Automatic Data Optimization (ADO)

1.3.3.5 Row-Level Compression Tiering

1.3.3.6 Segment-Level Compression Tiering

1.3.3.7 In-Database Archiving

1.5.4.1 Oracle ASM Disk Scrubbing

1.5.6.5 Real-Time Apply is Default Setting for Data Guard

1.6.1.4 Resource Manager Runaway Query Management

1.6.1.5 Spot ADDM

1.6.2.4 Database Replay Workload Scale-Up and Characterization

1.7.3.1 Oracle Flex Cluster

1.8.3.1 Multi-Process Multi-Threaded Oracle

1.9.2.11 Resource Role Default Privileges

The important documents to read are :
1.  New Features Guide
2.  SQL Tuning Guide
3.  Database Testing Guide
4.  Administrator's Guide section on Managing a Multitenant Environment

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