SQL中TRUNCATE和DELETE的区别是什么?
如果你的答案是针对特定平台的,请注明。
SQL中TRUNCATE和DELETE的区别是什么?
如果你的答案是针对特定平台的,请注明。
当前回答
如果将TRUNCATE包装在事务中,则可以回滚。
请参阅下面的两个参考资料并自行测试:-
http://blog.sqlauthority.com/2007/12/26/sql-server-truncate-cant-be-rolled-back-using-log-files-after-transaction-session-is-closed/
http://sqlblog.com/blogs/kalen_delaney/archive/2010/10/12/tsql-tuesday-11-rolling-back-truncate-table.aspx
截断与删除是SQL面试中最臭名昭著的问题之一。一定要向面试官解释清楚,否则可能会让你丢掉这份工作。问题是没有多少人意识到这一点,所以如果你告诉他们YES Truncate可以回滚,他们很可能会认为答案是错误的。
其他回答
TRUNCATE是快的,DELETE是慢的。
尽管TRUNCATE没有问责制。
对于SQL Server或MySQL,如果有一个带有自动递增的PK, truncate将重置计数器。
DROP The DROP command removes a table from the database. All the tables' rows, indexes and privileges will also be removed. No DML triggers will be fired. The operation cannot be rolled back. TRUNCATE TRUNCATE removes all rows from a table. The operation cannot be rolled back and no triggers will be fired. As such, TRUNCATE is faster and doesn't use as much undo space as a DELETE. Table level lock will be added when Truncating. DELETE The DELETE command is used to remove rows from a table. A WHERE clause can be used to only remove some rows. If no WHERE condition is specified, all rows will be removed. After performing a DELETE operation you need to COMMIT or ROLLBACK the transaction to make the change permanent or to undo it. Note that this operation will cause all DELETE triggers on the table to fire. Row level lock will be added when deleting.
来自:http://www.orafaq.com/faq/difference_between_truncate_delete_and_drop_commands
通过发出TRUNCATE TABLE语句,您将指示SQL Server删除表中的每条记录,而不进行任何日志记录或事务处理。
对原始答案的一个小修正——删除也会产生大量的重做(因为undo本身是由重做保护的)。这可以从autotrace输出中看到:
SQL> delete from t1;
10918 rows deleted.
Elapsed: 00:00:00.58
Execution Plan
----------------------------------------------------------
0 DELETE STATEMENT Optimizer=FIRST_ROWS (Cost=43 Card=1)
1 0 DELETE OF 'T1'
2 1 TABLE ACCESS (FULL) OF 'T1' (TABLE) (Cost=43 Card=1)
Statistics
----------------------------------------------------------
30 recursive calls
12118 db block gets
213 consistent gets
142 physical reads
3975328 redo size
441 bytes sent via SQL*Net to client
537 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
2 sorts (memory)
0 sorts (disk)
10918 rows processed