The DBA_DML_LOCKS view in Oracle provides information about all the data manipulation language (DML) locks in the database. This view is useful for monitoring and diagnosing locking issues related to DML operations such as INSERT, UPDATE, and DELETE.
Description of DBA_DML_LOCKS View
The DBA_DML_LOCKS view contains information about DML locks that are currently held in the database.
Columns in DBA_DML_LOCKS View
Some of the important columns in the DBA_DML_LOCKS view include:
- SESSION_ID: The ID of the session holding the lock.
- OWNER: The owner of the object being locked.
- NAME: The name of the object being locked.
- TYPE: The type of the object being locked (e.g., TABLE, INDEX).
- MODE_HELD: The mode in which the lock is held (e.g., ROW EXCLUSIVE (RX), SHARE (S)).
- MODE_REQUESTED: The mode in which the lock is requested.
Example Usage
Here’s an example query to retrieve information from the DBA_DML_LOCKS view:
SELECT SESSION_ID, OWNER, NAME, -- TYPE, MODE_HELD, MODE_REQUESTED FROM DBA_DML_LOCKS ORDER BY SESSION_ID;
Here is the OUTPUT
Detailed Example
Suppose you want to find out which sessions are holding DML locks on a specific table, and the mode of these locks:
SELECT SESSION_ID, OWNER, NAME, -- TYPE, MODE_HELD, MODE_REQUESTED FROM DBA_DML_LOCKS WHERE NAME = 'MY_TABLE' AND OWNER = 'MY_SCHEMA';
Checking for Blocking DML Locks
To identify sessions that are blocking others with DML locks, you can combine DBA_DML_LOCKS with other views such as V$SESSION:
SELECT s1.SID AS blocking_session, s1.SERIAL# AS blocking_serial, s2.SID AS waiting_session, s2.SERIAL# AS waiting_serial, dl.OWNER, dl.NAME, dl.MODE_HELD, dl.MODE_REQUESTED FROM DBA_DML_LOCKS dl JOIN V$SESSION s1 ON dl.SESSION_ID = s1.SID JOIN V$SESSION s2 ON s1.SID = s2.BLOCKING_SESSION WHERE s2.EVENT = 'enq: TX - row lock contention';
Conclusion
The DBA_DML_LOCKS view is an important tool for database administrators to monitor and diagnose issues related to DML operations in the Oracle database. It provides detailed information about DML locks held by sessions, which is crucial for resolving locking and contention issues.
By using DBA_DML_LOCKS, along with other dynamic performance views such as V$SESSION, administrators can effectively manage and troubleshoot locking problems, ensuring smooth database operations.