當一個row update正在進行時，此row 可被查詢，原始資料存放在rollback segment，
此row 可被查詢便是處於row share 狀態，其他使用者可以讀取或修改table資料，但
被阻止使用alter table..更改表格結構，或drop table 動作
row exclusive lock: 當一個session update某row 後，仍未commit，或rollback
其餘update此row的session 會無法下commit動作，可避免兩個使用者同時更新同一筆
資料
(1z0-042 p43)
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| View | Description | |||
| V$LOCK | Lists the locks currently held by Oracle Database and outstanding requests for a lock or latch | |||
| DBA_BLOCKERS | Displays a session if it is holding a lock on an object for which another session is waiting | |||
| DBA_WAITERS | Displays a session if it is waiting for a locked object | |||
| DBA_DDL_LOCKS | Lists all DDL locks held in the database and all outstanding requests for a DDL lock | |||
| DBA_DML_LOCKS | Lists all DML locks held in the database and all outstanding requests for a DML lock | |||
| DBA_LOCK | Lists all locks or latches held in the database and all outstanding requests for a lock or latch | |||
| DBA_LOCK_INTERNAL | Displays a row for each lock or latch that is being held, and one row for each outstanding request for a lock or latch |
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