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Which statements are correct regarding indexes？ （）
A: When a table is dropped， the corresponding indexes are automatically dropped.
B: For each DML operation performed， the corresponding indexes are automatically updated.
C: Indexes should be created on columns that are frequently referenced as part of an expression.
D: A non-deferrable PRIMARY KEY or UNIQUE KEY constraint in a table automatically creates a uniqueindex.

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出自：Oracle认证

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