A)They are always created automatically.
B)They are always created manually.
C)They are generated If statistics_level is set to basic.
D)They are generated If statistics_level is set to typical.
E)They are always retained forever.
F)They can be retained forever.
G)They are generated if statistics_level Is set to all.
Answer: DFG
赵：
Statistics_level：basic 禁用，typical 正常，all 全部。
原文：https://blog.51cto.com/13854012/2425525