How to understand MySQL query execution plan and optimizer?

How to understand MySQL query execution plan and optimizer?

Overview:
MySQL is one of the most commonly used open source relational databases. Its query execution plan and optimizer are the key to MySQL query performance optimization. Understanding and understanding MySQL's query execution plan and optimizer can help us optimize query statements and improve database performance. This article will introduce the concepts of MySQL query execution plan and optimizer, and show how to analyze and optimize queries through code examples.

1. Query execution plan
The query execution plan is the roadmap for executing query statements in MySQL. It describes how MySQL chooses the best path to execute the query. Through the query execution plan, we can understand the decisions made by MySQL's query optimizer during the execution of the query.

MySQL query execution plan can be obtained by using the EXPLAIN keyword. The following is the execution plan of an example query statement:

EXPLAIN SELECT * FROM table_name WHERE condition;

By executing the above statement, we can obtain the query execution plan and see how MySQL processes the query. . The results of the query execution plan include the following important columns:

1. id: the identifier of the query, each query has a unique identifier;
2. select_type: query Type, such as simple query, joint query, subquery, etc.;
3. table: the table involved in the query;
4. type: the access type of the table, such as full table scan, index scan, etc.;
5. possible_keys: indexes that may be used;
6. key: actual indexes used;
7. rows: the expected number of rows to be scanned;
8. Extra: extra information, such as the use of temporary tables, use of file sorting, etc.

By analyzing the query execution plan, we can determine whether an index needs to be created in the query statement, whether query conditions need to be optimized, whether JOIN needs to be used, etc.

2. Optimizer
MySQL's optimizer is a very complex part. It is responsible for selecting the optimal query execution plan to improve query performance. The optimizer will consider multiple factors when executing a query statement, such as table structure, index selection, complexity of query conditions, etc.

The optimizer will select the optimal query execution plan based on the characteristics of the query statement and the statistical information of the database. It analyzes each part of the query statement, selects appropriate indexes and JOIN methods, and minimizes IO operations and CPU overhead.

The working principle of the optimizer is very complex and contains many optimization algorithms and rules. In actual use, we can affect the optimizer's decision-making by adjusting the MySQL configuration file.

3. Code Example
The following is an example to illustrate how to analyze the query execution plan and optimize the query statement. Suppose there is a student table students and a course table courses, and they have a foreign key relationship. We need to query the information of students taking a certain course in the curriculum:

SELECT s.name, s.age
FROM students s
JOIN courses c ON s.id = c.student_id
WHERE c.course_name = 'math';

By executing the above query statement, we can get the query execution plan. Assume that the result of the query execution plan shows that the type column is ALL, that is, a full table scan. This shows that MySQL does not use indexes and needs to optimize the query statement.

In order to improve query performance, we can create an index:

CREATE INDEX idx_student_id ON courses (student_id);

Then execute the query statement again and view the query execution plan. If the type column changes to ref or index, it indicates that the index is used.

In addition to creating indexes, we can also optimize query conditions. For example, use covering indexes to reduce the number of table accesses, or rewrite query statements, change the JOIN order, etc.

Through this example, we can see how to use query execution plans and optimizers to improve query performance.

Summary:
MySQL’s query execution plan and optimizer are important tools for optimizing query performance. Knowing and understanding MySQL's query execution plans and optimizer is crucial to improving database performance. By analyzing the query execution plan, we can understand the execution path of the query and optimize the query statement based on the results. The optimizer is responsible for selecting the best execution plan and improving query performance. In actual use, we should continuously improve database performance by continuously analyzing execution plans and optimizing query statements.

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