Mysql's in does not use the index

MySQL is a powerful relational database management system that uses indexes to speed up data retrieval, thereby improving database performance. However, in some cases, MySQL's in query statement may not use the index, which will cause a serious decrease in performance.

This article will discuss the reasons why in query statements in MySQL do not use indexes, and introduce how to improve performance by optimizing query statements and establishing appropriate indexes.

1. How the in query statement works

In MySQL, the in query statement is used to determine whether a target value exists in a specified value list. For example:

SELECT * FROM table WHERE id IN (1, 2, 3);

This query statement will return all rows with id 1, 2 or 3 in the table.

During query execution, MySQL will execute a corresponding query for each value in the list, and then merge the results together. If the list contains a large number of values, this process can be very time-consuming, affecting query performance.

In order to improve query efficiency, MySQL usually uses indexes to speed up in queries. An index is a special data structure that helps MySQL quickly find the required data rows. When MySQL executes an in query, if the index covers all columns of the query, MySQL can use this index to speed up the query. This can greatly reduce the cost of in queries.

2. The reason why the in query statement does not use the index

However, when MySQL uses the in query statement, it is not always able to use the existing index. The reasons why MySQL does not use indexes usually include the following:

2.1 The value list is too long

MySQL will compare each value in the list with the index when executing an in query to determine Whether this value is in the index. Therefore, if the list of values ​​is too long, MySQL will not use the index and will instead use a full table scan to execute the query. This can cause a drastic drop in query performance, especially when the amount of data in the table is very large.

2.2 The value list does not match the index data type

Another reason is that MySQL cannot use the index when the data type in the value list does not match the data type of the index field. For example, if the indexed field is of type int but the list of values ​​contains values ​​of type string or date, MySQL will not be able to use the index to execute the query.

2.3 There are NULL values ​​in the value list

If the value list contains NULL values, MySQL will not be able to use the index to execute the query. The reason is that indexes cannot contain NULL values, so MySQL needs to perform a full table scan to find rows that contain NULL values.

2.4 The value list is not at the prefix position of the index

MySQL only matches the values ​​in the value list at the prefix position of the index. If the value of the value list is not in the prefix position of the index, MySQL will not be able to use the index to speed up the query.

3. How to optimize in query performance

In order to avoid the situation where in query does not use the index, some optimization measures need to be taken to improve query performance. The following are some optimization suggestions:

3.1 Limit the length of the value list

If you need to use the in query statement, you can limit the length of the value list to ensure that MySQL can use the index to execute the query. Specifically, you could try breaking up the list of values ​​into multiple smaller lists of values ​​and then using the UNION ALL operation to merge the results together. This enables MySQL to use the index for each small list of values.

3.2 Use EXISTS instead of IN

EXISTS is an alternative to IN queries that can greatly improve query performance. Specifically, the in query can be converted into an exists query, such as:

SELECT * FROM table WHERE EXISTS (SELECT 1 FROM table2 WHERE table.id = table2.id);

This query checks whether there are matching rows between the two tables. If MySQL could use an index to retrieve these rows, the time to execute the query would be significantly reduced.

3.3 Ensure that the data type of the value list matches the data type of the index field

Another optimization suggestion is to ensure that the data type of the value list matches the data type of the index field. If there is a mismatch, you can use a type conversion function to force a match, such as the CAST or CONVERT functions.

3.4 Avoid including NULL values ​​in the value list

To avoid MySQL being unable to use the index, do not include NULL values ​​in the value list. If you need to query rows containing NULL values, use the IS NULL or IS NOT NULL operator.

3.5 Ensure that the values ​​in the value list are at the prefix position of the index

Finally, you can try to move the values ​​in the value list to the prefix position of the index to ensure that MySQL can use the index to perform queries . You can use the ORDER BY clause and the LIMIT clause to control the order and length of the result set so that MySQL can use the index to execute the query.

4. Summary

Due to how the in query statement works and how it is used, MySQL is not always able to use indexes to speed up in queries. To avoid this problem, you can use the above optimization suggestions to improve query performance.

In general, optimizing the performance of MySQL query statements requires understanding the internal mechanism and optimization techniques of MySQL. By optimizing indexes and query statements, MySQL's performance and scalability can be greatly improved, allowing it to process massive data more effectively.

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