What is a covering index in MySQL?

Overriding indexes can significantly improve MySQL query performance. 1) Overwrite index is defined as an index containing all columns required for query, reducing I/O operations. 2) Its working principle uses the B-Tree structure to directly obtain data from the index to avoid returning to tables. 3) Basic usages such as SELECT username, email FROM users WHERE username = 'alice', advanced usages can be used for complex query and aggregation operations.

introduction

When we talk about database optimization, covering index is undoubtedly an exciting topic. It is like a superhero of database queries, which can significantly improve query performance. In this post, I will take you into the magical power of overlay indexing in MySQL. We will not only explore its definition and working principle, but also share some practical usage examples and performance optimization tips. After reading this article, you will learn how to use overlay indexes to make your database query more efficient.

Review of basic knowledge

Let's first review the basic concepts related to overlay indexes. In MySQL, indexing is a very important concept, which can speed up data retrieval. Common index types include B-Tree index, full-text index and hash index, etc. Overwriting indexes is based on further optimization of these indexes, which allows queries to get all the data they need only through the index without having to go back to the table (i.e. access to the data table itself).

Core concept or function analysis

Definition and function of overlay index

Overlay index, as the name implies, is the index that can overwrite all columns required for a query. When a query only needs to read data from the index and does not need to access the table itself, it is called an overwrite query. The advantage of covering indexes is that it reduces I/O operations, thereby improving query performance.

To give a simple example, suppose we have a table called employees , which contains three columns: id , name and department . We create an index INDEX idx_name_dept (name, department) and then execute the following query:

 SELECT name, department FROM employees WHERE name = 'John';

In this query, MySQL can directly get the value of name and department from the idx_name_dept index without accessing the employees table itself, which is the override query.

How overlay index works

The working principle of overlay index can be understood from the following aspects:

• Index structure : Overlay index utilizes the structure of the B-Tree index. In the B-Tree index, each node contains not only key values, but also values ​​of other columns. When the index contains all the columns required for the query, the data can be obtained directly from the index.
• Avoid returning to tables : Traditional queries may need to find row pointers through the index, and then access the data in the table through the row pointers (return to tables). Overwriting the index avoids this step, reading data directly from the index, reducing I/O operations.
• Performance improvement : Due to reduced I/O operations, overwriting indexes can significantly improve query performance, especially in the case of large data volumes.

Let's look at a more complex example, suppose we have an order table orders containing four columns: id , customer_id , order_date and total_amount . We create an index INDEX idx_customer_order (customer_id, order_date, total_amount) and then execute the following query:

 SELECT customer_id, order_date, total_amount 
FROM orders 
WHERE customer_id = 123 AND order_date >= '2023-01-01';

In this query, MySQL can directly obtain the values ​​of customer_id , order_date and total_amount from idx_customer_order index without accessing orders table itself.

Example of usage

Basic usage

Let's look at a basic override index usage. Suppose we have a user table users , including id , username and email columns. We create an index INDEX idx_username_email (username, email) and then execute the following query:

 SELECT username, email FROM users WHERE username = 'alice';

In this query, MySQL can directly get username and email values ​​from idx_username_email index without accessing the users table itself.

Advanced Usage

Advanced usage of overriding indexes can help us handle more complex queries. Suppose we have a product table products contains four columns: id , name , category and price . We create an index INDEX idx_category_price (category, price) and then execute the following query:

 SELECT category, AVG(price) 
FROM products 
WHERE category = 'Electronics' 
GROUP BY category;

In this query, MySQL can directly get the values ​​of category and price from idx_category_price index without accessing products table itself. Since the index contains price column, we can directly perform aggregation operations on the index to further improve query performance.

Common Errors and Debugging Tips

When using overlay indexes, you may encounter some common problems and misunderstandings:

• Indexed column order : It is very important to override the column order of the index. If the order of query criteria and selected columns do not match the order of index columns, MySQL may not be able to use overwrite indexes. For example, if we have an index INDEX idx_name_dept (name, department) but the query is SELECT department, name FROM employees WHERE name = 'John' , MySQL may not be able to use overwrite indexes.
• Index Maintenance : Overwriting indexes increases the maintenance cost of indexes because each time data is inserted, updated, or deleted, the index needs to be updated. If the index is too large, it may affect the performance of the write operation.

Methods to debug these problems include:

• Use EXPLAIN : Use the EXPLAIN statement to view the plan of MySQL to execute queries, helping us understand whether the overlay index is used. For example:

 EXPLAIN SELECT name, department FROM employees WHERE name = 'John';

• Adjust the order of index columns : Adjust the order of index columns according to the actual needs of the query to ensure that the overlay index can be used effectively.

Performance optimization and best practices

In practical applications, how to optimize the use of overlay indexes? Let's explore some performance optimizations and best practices:

• Select the appropriate column : When selecting a column that overwrites the index, consider the frequency and data volume of the query. Selecting columns that often appear in the query can maximize the effect of overwriting the index.
• Avoid over-index : While overwriting indexes can improve query performance, excessive indexes can increase maintenance costs. A balance needs to be found between query performance and write performance.
• Monitor and adjust : Regularly monitor query performance and adjust the index structure according to actual conditions. For example, you can use SHOW INDEX statement to view the current index situation:

 SHOW INDEX FROM employees;

• Code readability and maintenance : When writing queries, pay attention to the readability and maintenance of the code. Use meaningful column names and index names to add comments to explain the purpose and logic of the query.

Through these methods, we can make full use of the advantages of overlay indexes to improve the query performance of MySQL database. I hope this article can help you better understand and apply coverage indexes, making your database queries more efficient.

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