mysql whether to change table lock table

When MySQL modifys table structure, metadata locks are usually used, which may cause the table to be locked. To reduce the impact of locks, the following measures can be taken: 1. Keep tables available with online DDL; 2. Perform complex modifications in batches; 3. Operate during small or off-peak periods; 4. Use PT-OSC tools to achieve finer control.

MySQL table lock: a double-edged sword

When MySQL modifying the table structure, locks will be used. This thing sounds simple, but it is actually a lot of tricks to use. How do you ask how to change the table to avoid locking the table? It's not that simple, it depends on the situation. To say "not locking the watch" directly is a hooligan, and you have to break it apart and talk about it.

In this article, we will not play with virtual things. We will directly analyze the lock mechanism when MySQL modifying the table structure, and how to minimize the impact of locks so that you can lose some hair. After reading it, you can understand the principle behind locking the table and how to handle it gracefully in practical applications.

Let’s talk about the basics first. There are many types of MySQL locks and are closely related to the InnoDB engine. When modifying the table structure, metadata locks are usually used, which will block the modification of the table structure. There are also row locks, page locks, table locks, etc. They will automatically lock according to your operations and engine selection. Don't be scared by these nouns. The key is to understand how they affect your operation of modifying table structure.

Operations that modify table structure, such as ALTER TABLE , actually do a lot of things internally: adding locks, modifying metadata, and releasing locks. This process has a huge impact on performance. Imagine a high-concurrency system, if you ALTER TABLE , the entire table is stuck, and the user accesses are all kneeling, that scene... Therefore, it is very important to refine the control lock.

Let's take a look at a simple example and experience the power of ALTER TABLE :

 <code class="sql">ALTER TABLE my_table ADD COLUMN new_column INT;</code>

This line of code seems simple, but MySQL does a lot of things behind it. It will first acquire the table lock, then modify the table structure, and finally release the lock. If your my_table is large, or the concurrency is high, this process may last for a long time, causing application blockage.

So, how to optimize? There is no silver bullet, but there are some tips to try:

• Online DDL: After MySQL 5.6, the online DDL function was introduced, which allows the table to be kept available while modifying the table structure. This is like building a road on a highway, trying not to completely close the road, but just building it in sections. Of course, online DDL is not omnipotent, it will add some overhead, and some complex modification operations may still require locking tables.
• Batch modification: If your modification operation is more complicated, you can consider doing it in batches. For example, you need to add multiple columns, which can be divided into multiple ALTER TABLE operations to complete, which can reduce the time of each table lock.
• Small table priority: If your table is relatively small and the impact of locking tables is relatively small, you can directly use the traditional ALTER TABLE .
• Non-peak operation: Selecting a time period with low database usage for table structure modification can minimize the impact on the business.
• PT-OSC: The PT-OSC tool in Percona Toolkit can help you modify table structure online and provide finer granular control. It is more powerful than the online DDL that comes with MySQL, but it is also more complicated to use.

Finally, remember one thing: there is no perfect solution. Which method to choose depends on your specific situation. It should be considered comprehensively based on factors such as the scale of the table, concurrency volume, and business needs. Don’t expect to be done once and for all. Continuous monitoring and optimization are the king. Don’t forget that code is dead, people are alive, and flexible application is the key. Only by practicing and summarizing more can one become a true MySQL expert.

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