Reasons why tables are locked in Oracle and how to deal with them

Causes and processing methods of table locking in Oracle

In Oracle database, table locking is a common phenomenon, and the reasons for table locking are There are many kinds. This article will explore some common reasons why tables are locked, and provide some processing methods and related code examples.

1. Types of locks

In Oracle database, locks are mainly divided into shared locks (Shared Lock) and exclusive locks (Exclusive Lock). Shared locks are used for read operations and allow multiple sessions to read the same resource at the same time, while exclusive locks only allow one session to write to the resource at a time.

2. Reasons why the table is locked

The reasons why the table is locked mainly include:

• The transaction is not submitted: A transaction adds an exclusive lock to the table, but Uncommit or rollback prevents other sessions from accessing the table.
• Long-running queries: A certain query statement takes too long to execute and locks the table, causing other concurrent operations to be affected.
• Deadlock: A deadlock occurs between multiple sessions, causing one of the sessions to be unable to continue execution.
• Database connection leak: A database connection is not properly closed or the lock resource is released, resulting in the table being unable to be accessed by other sessions.

3. Processing method

3.1 View lock information

In Oracle, we can view the lock information under the current session through the following SQL statement:

SELECT 
    c.owner, c.object_name, c.object_type, b.SID, b.serial#, b.status, b.osuser
FROM 
    v$locked_object a , v$session b, dba_objects c
WHERE 
    b.sid = a.session_id AND a.object_id = c.object_id;

3.2 Unlock operation

3.2.1 Release the lock for a specific session

ALTER SYSTEM KILL SESSION 'SID,SERIAL#';

3.2.2 Release the table-level lock

ALTER TABLE table_name ENABLE ROW MOVEMENT;
ALTER TABLE table_name DISABLE ROW MOVEMENT;

3.2.3 Force the lock to be revoked

ALTER SYSTEM KILL SESSION 'SID,SERIAL#' IMMEDIATE;

3.3 Avoid table locks

• Use appropriate transaction isolation levels to control locking of tables.
• Regularly check the database connection status and close unused connections in a timely manner.
• Avoid long-running queries and optimize SQL statements and index design.

Conclusion

Table locking is a common problem in Oracle database. It is very important to understand the reasons for table locking and how to deal with it. Through proper monitoring and management, the impact of table lock problems on the system can be effectively avoided.

I hope this article can help readers better understand the problem of table locks in Oracle and master the corresponding processing methods.

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