How to improve performance by optimizing the MySQL query cache

MySQL is a popular open source database management system that is widely used in many websites and applications. One of the important performance improvement mechanisms is query caching. Query caching is the mechanism MySQL uses to cache the result set of a SELECT statement. When a query is cached, MySQL will store the result set in memory and return the cached results when the same query is requested again, rather than executing the query again. Under ideal circumstances, query caching can greatly improve query performance. However, if the query cache is not properly configured and optimized, it can have a negative impact on performance. Therefore, this article will explain how to improve performance by optimizing the MySQL query cache.

1. Configure query cache
   To enable query cache, you need to set the query cache size and enable the cache function in the MySQL configuration file. Add the following configuration in the My.cnf file:

query_cache_type = 1
query_cache_size = 128M

query_cache_type specifies the cache type. A value of 1 means query caching is enabled; a value of 0 means query caching is disabled.

query_cache_size specifies the size of the cache. It specifies the total size of all query result sets that can be cached. Its value should be adjusted based on system memory and load requirements. If the cache is too small, not all result sets will be cached. If the cache is too large, memory is wasted and performance may suffer.

2. Avoid long-running queries
   Query caching only applies to SELECT queries, and cached results will only be returned if the query result set has not changed. Therefore, if the query result set changes frequently, the effectiveness of the query cache will be greatly reduced. Long-running queries can also cause query caching to become less efficient because the cached results need to be kept in memory, and if the results in the cache are stale, the query needs to be re-executed to update the cache. Therefore, long-running queries should be avoided whenever possible to improve caching efficiency.
3. Avoid writing different queries
   If there are multiple queries with the same SQL statement and query parameters, you can improve query cache efficiency by using prepared statements. Prepared statements allow binding parameters by name instead of sending the same parameters through a copy of the SQL statement. In this way, multiple SQL queries can be merged into one, and only one query result set needs to be cached in the query cache.
4. Disable unnecessary query cache clearing
   The query result set needs to be cleared when the data table is modified, which usually occurs when INSERT, UPDATE and DELETE statements are executed. However, if the cache is always cleared, cache performance will be wasted. Therefore, you should choose to clear only necessary caches and disable unnecessary query cache clearing. Query caching can be disabled by adding the SQL_NO_CACHE keyword to the SQL query.
5. Enable query cache statistics
   MySQL provides query cache statistics, which can be used to understand query cache usage. You can use the SHOW STATUS command to view query cache statistics. These statistics can provide information about cache hit ratio, cache usage size, etc. This information can help identify performance bottlenecks in the query cache and examine the effectiveness of cache usage.
6. Choose appropriate cache items
   By default, MySQL caches the result sets of all SELECT statements, even if the result set is very small. However, this can waste cache because most small result sets may never be used again. To avoid this waste, you can choose whether the result set should be cached by using the SQL_CACHE or SQL_NO_CACHE keyword during the query.
7. Update query cache
   When the database table changes, the corresponding query results in the query cache also need to be updated. However, this process requires a lot of CPU and memory resources. To avoid this situation, check and refresh techniques can be used to update the query cache. This technology allows MySQL to update only the portion of the query cache that needs to be updated, rather than clearing the entire cache.

In short, optimizing the MySQL query cache is the key to improving performance. Query performance can be improved by configuring the appropriate query cache size, avoiding long-running queries, using prepared statements, disabling unnecessary query cache clearing, enabling query cache statistics, selecting appropriate cache entries and updating the query cache, and Reduce overhead.

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