Explain MySQL Query Cache (and why it's often disabled/deprecated).

MySQL query cache is often disabled or even marked as deprecated because it performs poorly in environments with high concurrency and frequent data updates. 1) Query cache improves performance by storing the results of SELECT statements, but depends on data stability. 2) In modern MySQL versions, query cache has been abandoned, and alternatives such as InnoDB buffer pooling, query rewriting and index optimization are recommended.

introduction

In database performance optimization, MySQL's query cache (Query Cache) has always been a topic of concern. Today we will dive into the mechanisms of MySQL query caching and why in modern MySQL versions it is often disabled or even marked as deprecated. Through this article, you will learn how query caching works, its pros and cons, and how to trade off using it in real-world applications.

Review of basic knowledge

MySQL query cache is a mechanism for storing the results of SELECT statements. When a SELECT query is executed, MySQL checks whether the results of the same query already exist in the query cache. If it exists, MySQL will return the cached result directly, rather than re-execute the query. This can significantly improve query performance, especially in scenarios where the same query is frequently executed.

However, the validity of the query cache depends on the stability of the data. If the data changes frequently, the cache hit rate will be greatly reduced and may even lead to performance degradation.

Core concept or function analysis

Definition and function of MySQL query cache

The core role of MySQL query cache is to reduce the load on the database by storing and reusing query results. It is suitable for queries whose results do not change over time, such as those with static data.

Let's look at a simple example:

 -- Suppose we have a simple query SELECT * FROM users WHERE status = 'active';

If the result of this query is cached, the next time the same query is executed, MySQL will return the result directly from the cache instead of accessing the disk again.

How it works

When a SELECT query is executed, MySQL generates a hash value of a query and looks for this hash value in the query cache. If a matching hash is found, MySQL checks whether the cached query is still valid (i.e. the data has not been modified). If valid, MySQL will directly return the cached result.

However, query cache maintenance costs are high. Whenever the data of the table changes, MySQL must invalidate the cache of all related queries. This means that query caches can become a performance bottleneck in environments with high concurrency and frequent data updates.

Example of usage

Basic usage

Enabling query caching is very simple, just set query_cache_type and query_cache_size in MySQL configuration file:

 -- Enable query cache SET GLOBAL query_cache_type = ON;
SET GLOBAL query_cache_size = 16M;

After setting this, MySQL will automatically try to cache query results that meet the criteria.

Advanced Usage

In some cases, you may want to disable cache for specific queries, or enable cache only for certain queries. This can be achieved through SQL_HINTS:

 -- Disable cache for specific queries SELECT SQL_NO_CACHE * FROM users WHERE status = 'active';

-- Forced cache SELECT SQL_CACHE * FROM users WHERE status = 'active';

This method can help you control the use of query cache more carefully and avoid unnecessary cache failures.

Common Errors and Debugging Tips

A common mistake is the mistake of thinking that query caches always improve performance. In fact, querying caches may result in more overhead if data changes frequently, because each data update requires invalidating the relevant cache.

When debugging query cache issues, you can use the following command to view the cache usage:

 SHOW STATUS LIKE 'Qcache%';

This will display the hit rate, number of inserts, number of failures and other information of the query cache, helping you judge the validity of the query cache.

Performance optimization and best practices

In practical applications, the following aspects need to be considered for the performance optimization of query cache:

• Data stability : If the data changes frequently, the effect of query cache will be greatly reduced. In this case, disabling query caching may be a better option.
• Query complexity : For complex queries, the cached results may be large and occupy a lot of memory. In this case, the benefits of cache and the use of memory need to be weighed.
• Concurrency : In high concurrency environments, the maintenance cost of query caches will increase significantly, which may lead to performance degradation.

In modern MySQL versions, query caches have been marked as deprecated (starting with MySQL 8.0), mainly because they perform poorly in environments with high concurrency and frequent data updates. Alternatives include buffer pooling using InnoDB, query rewriting, index optimization, etc. These methods usually provide better performance and scalability in modern database applications.

Overall, MySQL query caching is a useful tool, but it needs to be carefully evaluated when using it. By understanding how it works and limitations, you can better decide whether to enable query caching in your app.

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