What situations in Oracle database can cause tables to be locked?

What situations in Oracle database will cause the table to be locked?

In Oracle database, it is a common situation that the table is locked, which is usually caused by users performing data operations. Common table lock situations include row-level locking, transaction-level locking, and DDL operations. These situations will be introduced in detail below and corresponding code examples will be given.

1. Row-level locking:
   When the user updates the table in a transaction, the database will lock the corresponding data row to prevent other transactions from modifying the data at the same time. If the transaction is not committed or rolled back, other transactions will not be able to modify the data, causing the table to be locked. The following is a simple row-level locking example:

-- 事务1
BEGIN
    UPDATE employees
    SET salary = salary * 1.1
    WHERE department_id = 10
    FOR UPDATE;

    COMMIT;
END;

-- 事务2
BEGIN
    UPDATE employees
    SET salary = salary * 1.2
    WHERE employee_id = 100;

    -- 此时因为employee_id=100被事务1锁定，导致事务2无法执行
END;

2. Transaction-level locking:
   In addition to row-level locking, the entire transaction will also cause the table to be locked. Data operations on a table are performed within a transaction. Until the transaction commits or rolls back, other transactions will not be able to modify the table. The following is an example of transaction-level locking:

-- 事务1
BEGIN
    UPDATE employees
    SET salary = salary * 1.1
    WHERE department_id = 10;

    -- 由于使用了事务，整个employees表会被锁定，其他事务无法修改数据
    COMMIT;
END;

3. DDL operations:
   When performing DDL operations (such as ALTER TABLE, DROP TABLE, etc.), the table will be locked to avoid other transactions Perform data operations on the table until the DDL operation is completed. The following is an example of a DDL operation causing a table to be locked:

-- 事务1
BEGIN
    ALTER TABLE employees ADD COLUMN email VARCHAR2(100);

    -- 由于ALTER TABLE操作，employees表会被锁定，其他事务无法对表进行数据操作
    COMMIT;
END;

In short, table locking is a common database operation phenomenon. You need to pay attention to avoid unnecessary table locking when writing code to avoid unnecessary table locking. Improve the concurrency performance of the database.

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