Query optimization using EXPLAIN in MySQL

Usage and specific code examples of EXPLAIN in MySQL

1. Introduction

In MySQL, EXPLAIN is a very useful tool for analyzing the execution plan of query statements. It can help us understand how MySQL processes queries, and provides an important reference for optimizing query performance.

2. How to use

To use EXPLAIN for query analysis, you only need to put the query statement to be analyzed after the EXPLAIN keyword, for example ：

EXPLAIN SELECT * FROM users WHERE age > 18;

3. Interpretation results

EXPLAINThe analysis results are presented in the form of a table, including a number of columns, mainly including the following fields:

1. id: Indicates the query sequence number. If the query is nested, the id of the corresponding nesting level will also be incremented.
2. select_type: Indicates the type of query, including simple query, joint query, subquery, etc.
3. table: Indicates which table this row of data is associated with.
4. type: Indicates the way to access the table, including full table scan, index scan, range scan, etc.
5. possible_keys: Indicates indexes that may be used.
6. key: Indicates the actual index used.
7. key_len: Indicates the length of the index field.
8. ref: A constant or column representing a reference.
9. rows: Indicates the number of rows scanned.
10. Extra: Indicates other information, such as using temporary tables, file sorting, etc.

4. Optimization analysis

By analyzing the interpretation results, we can optimize performance. The following are some common optimization cases:

1. Index Optimization: Determine whether appropriate indexes are used by analyzing the possible_keys and key fields. If key is NULL, it means that no index is used, and you need to consider creating an index to optimize the query.
2. Optimization of the number of rows scanned: By analyzing the rows field, determine whether the number of rows scanned by the query is too many. If the number of scanned rows is large, consider rewriting the query to reduce the number of scanned rows.
3. Query type optimization: Determine the query type by analyzing the select_type field. For example, if the query is a subquery, you may want to consider using JOIN instead of the subquery to improve query performance.
4. Table association optimization: By analyzing the table field, determine whether there are multiple table associations and whether the order of table associations needs to be adjusted to reduce the use of temporary tables.

5. Sample code

The following is a sample code to illustrate how to use EXPLAIN:

EXPLAIN SELECT * FROM orders WHERE customer_id = 100;

Execute the above code, you can get Similar results:

+----+-------------+--------+-------+---------------+------+---------+------+------+-------+
| id | select_type | table  | type  | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+--------+-------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | orders | const | customer_id   | id   | 4       | const|1     | NULL  |
+----+-------------+--------+-------+---------------+------+---------+------+------+-------+

By observing this result, we can get the following information:

• The query is a simple query (SIMPLE).
• The query uses the customer_id index.
• The number of rows scanned by the query is 1.

Through this information, you can judge that this query has better performance because it uses an index and only scans one row.

6. Summary

By using EXPLAIN, we can analyze the execution plan of the query statement to optimize performance. We can judge whether the query needs to be optimized and decide which optimization strategies to adopt by analyzing the query type, index usage, number of scanned rows and other information. EXPLAIN is a very important tool in MySQL optimization, helping us understand MySQL's execution plan and improve query performance.

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