How to lock table in oracle

Oracle database is a very powerful relational database management system. In Oracle, table locking can provide protection for ongoing concurrent transactions. In some cases, it may be necessary to lock a table to ensure that the data in the table will not be modified by other users. This article mainly introduces how Oracle locks tables.

First of all, you need to understand Oracle's locking mechanism. Oracle provides two main locking mechanisms, one is exclusive lock and the other is shared lock. An exclusive lock allows only one transaction to write to the row, while a shared lock allows multiple transactions to read the row.

There are two ways to lock a table in Oracle. One is to use the ALTER TABLE statement, and the other is to use the LOCK TABLE statement. Let's take a look at the specific implementation of these two methods.

Use the ALTER TABLE statement to lock the table

Let’s look at the ALTER TABLE statement first. The ALTER TABLE statement can be used to change the structure of a table or its properties. In Oracle, it can also create locked tables.

ALTER TABLE table_name ADD PRIMARY KEY (col);

The statement here is used to add a primary key to the table. Once the primary key is added to the table, the table will automatically be locked and other users cannot Perform any operations involving primary keys. Of course, there are other types of constraints that can be used, such as FOREIGN KEY constraints, etc., which can also play a role in locking the table.

Adding primary keys or other types of constraints to a table actually implicitly locks the table, which is easy to operate and effective, but difficult to control flexibly.

Use the LOCK TABLE statement to lock the table

Using the LOCK TABLE statement to lock the table is more flexible and allows for more fine-grained control. The syntax is as follows:

LOCK TABLE table_name IN lock_mode NOWAIT;

Among them, table_name is the name of the table that needs to be locked, and lock_mode is the lock mode. There are four locking modes supported by Oracle, which are:

• SHARE MODE: shared lock, allowing other transactions to read the table.
• SHARE ROW EXCLUSIVE MODE: Shared lock row locking allows multiple transactions to read concurrently, but will lock rows that cannot be modified by other transactions.
• EXCLUSIVE MODE: Exclusive lock, which does not allow other transactions to read and modify the table.
• ROW EXCLUSIVE MODE: Exclusive row lock, allowing other transactions to read the table, but only the current transaction can modify the table.

Among them, NOWAIT means that if the table is already locked, there will be no waiting and an error will be returned directly. If NOWAIT is not used, it will wait until the table can be locked.

For example, we can use the following statement to lock a table:

LOCK TABLE employees IN EXCLUSIVE MODE NOWAIT;

This sentence locks the employees table and only allows the current transaction to proceed Modify operations. If another transaction attempts to read or modify the table, it will wait or report an error until the current transaction completes and releases the lock.

In database design, we should usually try to avoid frequent table locking, because it will lead to performance degradation and may lead to deadlocks and other problems. If the design and use of the table are unreasonable, or in some special circumstances, locking the table can become a necessary means.

Summary

This article introduces how to lock tables in Oracle. Oracle provides two main locking mechanisms, ALTER TABLE and LOCK TABLE. Each method has different advantages, disadvantages and uses. Table locking requires careful consideration. In actual applications, it should be selected and used according to specific situations to avoid deadlocks and other problems.

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