explain usage in mysql

Detailed explanation of EXPLAIN usage in MySQL and code examples

In MySQL, EXPLAIN is a very useful tool for analyzing the execution plan of query statements. By using EXPLAIN, we can understand how the MySQL database executes query statements, thereby helping us optimize query performance.

The basic syntax of EXPLAIN is as follows:

EXPLAIN SELECT 列名 FROM 表名 WHERE 条件;

The return result of EXPLAIN contains the following important fields:

• id: represents the identifier of the query, each Each query has a unique identifier.
• select_type: Indicates the type of query. Possible values ​​include SIMPLE, PRIMARY, SUBQUERY, DERIVED, UNION, UNION RESULT, etc.
• table: Indicates the table name of the query.
• partitions: Indicates the partitions used by the query.
• type: Indicates the access type. Possible values ​​include ALL, index, range, ref, eq_ref, const, system, NULL, etc. Generally speaking, the better the value of the access type, the better the query performance.
• possible_keys: Indicates possible indexes.
• key: Indicates the actual index used.
• key_len: Indicates the length of the index field.
• ref: Indicates the relationship between indexes.
• rows: Indicates the number of rows scanned.
• filtered: Indicates the degree of filtering of query results.
• Extra: Indicates additional information, such as whether a temporary table is used, file sorting is used, etc.

The following is a specific code example to illustrate how to use EXPLAIN by analyzing and optimizing the execution plan of a query statement.

Suppose we have a table named "orders" that stores information related to user orders, including order ID, user ID, order amount, etc.

We want to query the information of orders with an order amount greater than 1000, and sort them in descending order by order ID. The query statement is as follows:

SELECT * FROM orders WHERE amount > 1000 ORDER BY order_id DESC;

First, we can use EXPLAIN to analyze the execution plan of this query statement.

EXPLAIN SELECT * FROM orders WHERE amount > 1000 ORDER BY order_id DESC;

After executing EXPLAIN, MySQL will return the execution plan of the query. We can optimize query performance based on the returned results.

Assume that the returned execution plan is as follows:

id  select_type   table  type  possible_keys  key  key_len  ref  rows  Extra
1   SIMPLE        orders range amount        NULL NULL     NULL 1000  Using where;Using filesort

In the above execution plan, you can see that the value of type is "range", which means that MySQL will execute an index on an index in the table. Range scan. This means that MySQL is not using any indexes to speed up queries, resulting in poor query performance. At the same time, "Using filesort" in the Extra field indicates that file sorting is used, which will also have a negative impact on query performance.

In order to optimize query performance, we can add an index to the "amount" field:

ALTER TABLE orders ADD INDEX idx_amount (amount);

Execute EXPLAIN again, we can see that the execution plan has changed:

id  select_type   table  type   possible_keys  key       key_len  ref  rows  Extra
1   SIMPLE        orders range  idx_amount     idx_amount 2        NULL 1000  Using where

Now, the value of type changes to "range", indicating that MySQL will use the newly added index to perform a range scan. At the same time, there is no "Using filesort" prompt in the Extra field, indicating that query performance has been significantly improved.

Through the above examples, we can see how to use EXPLAIN and its importance. By analyzing the execution plan, we can find the bottlenecks that affect query performance and take corresponding optimization measures to improve the query efficiency of the database.

To sum up, using EXPLAIN can help us deeply understand the query execution process of the MySQL database and find out how to optimize query performance. By analyzing the execution plan, we can determine whether we need to add indexes, change the order of query statements, etc. In the actual development process, reasonable use of the EXPLAIN tool is one of the important links in improving database performance.

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