What is the Using filesort status in EXPLAIN and how to avoid it?

"Using filesort" means that MySQL does not use indexes when sorting, but uses file sorting, which will degrade query performance. Methods to avoid include: 1. Create an appropriate index, such as CREATE INDEX idx_last_name ON users(last_name); 2. Adjust the query to use the index field or overwrite the index.

introduction

When you execute a query in MySQL, you may use the EXPLAIN statement to view the execution plan of the query. This tool is like a query X-ray, helping you understand how MySQL handles your query. In these execution plans, you may see a state called "Using filesort". What does this state mean? What impact does it have on query performance? More importantly, how do we avoid it? This article will explore these issues in depth and provide practical solutions and optimization techniques.

Review of basic knowledge

In MySQL, the EXPLAIN statement is a tool used to analyze query execution plans. It will show how MySQL executes your SQL statements, including the index used, the order of joins of tables, etc. "Using filesort" is a state in the EXPLAIN output, indicating that MySQL does not use indexes when performing sorting operations, but uses file sorting, which usually results in performance degradation.

Sorting operations in MySQL usually occur in ORDER BY or GROUP BY statements. When MySQL can't use indexes to complete sorting, it uses file sorting, which means it needs to load data into memory or temporary files for sorting, which is obviously much slower than using index sorting directly.

Core concept or function analysis

Definition and function of "Using filesort"

"Using filesort" means that MySQL does not use indexes when performing sorting operations, but uses file sorting. This usually happens in ORDER BY or GROUP BY statements, and when MySQL cannot use indexes to complete sorting, it selects file sorting. Although this method can complete sorting, it is usually less performance than using index sorting due to the need for additional I/O operations.

For example, suppose we have a simple query:

 EXPLAIN SELECT * FROM users ORDER BY last_name;

If "Using filesort" is displayed in the EXPLAIN result, it means that MySQL does not use indexes to sort the last_name field, but uses file sorting.

How it works

When MySQL decides to use file sorting, it performs the following steps:

1. Read data : MySQL will read the rows that need to be sorted from the table.
2. Sort : These lines are loaded into memory or temporary files and then sorted.
3. Return result : After the sorting is completed, MySQL will return data based on the sorting result.

The performance bottleneck of file sorting is mainly in I/O operations, especially when the amount of data is large, which may lead to a large number of disk read and write operations, seriously affecting query performance.

Example of usage

Basic usage

Suppose we have a users table containing id , first_name and last_name fields. We want to query all users by last_name :

 EXPLAIN SELECT * FROM users ORDER BY last_name;

If the EXPLAIN result shows "Using filesort", we need to consider how to optimize this query.

Advanced Usage

To avoid "Using filesort", we can create an index to support sorting operations. For example:

 CREATE INDEX idx_last_name ON users(last_name);

After creating the index, execute EXPLAIN again:

 EXPLAIN SELECT * FROM users ORDER BY last_name;

If the index is created correctly, "Using filesort" should no longer be displayed in the EXPLAIN result.

Common Errors and Debugging Tips

• Error 1 : Non-indexed fields were used in ORDER BY.

  • Solution : Make sure that the fields in ORDER BY have indexes, or adjust the query to use the indexed fields.

• Error 2 : Complex sorting conditions were used, resulting in the inability to use the index.

  • Solution : Simplify sorting conditions, or consider using multiple column indexes to support complex sorting.

Performance optimization and best practices

To avoid "Using filesort", we need to optimize from the following aspects:

• Create a suitable index : Make sure that fields in ORDER BY or GROUP BY have indexes. Multi-column indexes can support complex sorting conditions.

• Adjust query : Sometimes you can avoid file sorting by adjusting the query. For example, if only partial fields are needed, overwrite indexes can be used.

• Performance comparison : Before and after optimization, use EXPLAIN and actual execution time to compare query performance. Here is a simple example:

 -- Before optimization EXPLAIN SELECT * FROM users ORDER BY last_name;

-- Optimized EXPLAIN SELECT * FROM users USE INDEX (idx_last_name) ORDER BY last_name;

By comparing the EXPLAIN results with the actual execution time, we can see the optimization effect.

• Best practice : Consider common query patterns when designing table structures and indexes. Try to avoid using non-index fields in ORDER BY or GROUP BY to ensure the efficiency and maintainability of the query.

In practical applications, avoiding "Using filesort" can not only improve query performance, but also reduce resource consumption and improve the response speed of the overall system. Through continuous optimization and practice, we can better master MySQL query optimization skills.

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