【多项选择题】
Which two statements about Flashback Query are true？（）
A、A.It is generated by using the redo log files.
B、B.It helps in row-level recovery from user errors.
C、C.It can be performed to recover ALTER TABLE statements
D、D.It fails when undo data pertaining to the transaction is overwritten.
E、E.The database has to be opened with the resetlogs option after performing Flashback Query.
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