truncate ,delete ,drop的区别
1.truncate和delete删除表中的数据；drop是将表从数据字典中删除，并清空表中的数据。2.truncate和drop 为DDL语句，不会产生重做日志；delete为DML语句，会产生重做日志。
3.truncate之后会降低高水位线，而delete之后不会降低高水位线。
4.truncate，drop，delete执行所花的时间不同。
SQL> set timing on
SQL> delete from test;
151358 rows deleted.
Elapsed: 00:00:06.16
SQL> rollback;
Rollback complete.
Elapsed: 00:00:07.00
SQL> drop table test;
Table dropped.
Elapsed: 00:00:00.36
SQL> flashback table test to before drop;
Flashback complete.
Elapsed: 00:00:00.02
SQL> drop table test purge;
Table dropped.
Elapsed: 00:00:00.06
SQL> truncate table test;
Table truncated.
Elapsed: 00:00:00.34
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