How to Read MySQL EXPLAIN?

Understanding how to interpret and analyze the output of the MySQL EXPLAIN statement is essential for database administrators, developers, and anyone involved in optimizing database performance.

The EXPLAIN statement provides insights into how MySQL executes queries and helps identify potential bottlenecks or inefficiencies in the query execution plans. By mastering the interpretation of the EXPLAIN output users can optimize queries, improve database performance, and enhance overall application efficiency.

The MySQL EXPLAIN is a powerful tool for analyzing and optimizing SQL queries. It provides insight into how MySQL executes a query including the details about the chosen execution plan the order in which tables are accessed and the use of the indexes. Understanding the output of the EXPLAIN can help database developers and administrators identify performance bottlenecks and optimize query performance.

The syntax of the EXPLAIN statement in MySQL is straightforward:

EXPLAIN SELECT * FROM table_name WHERE condition;

When you execute an SQL query prefixed with the EXPLAIN MySQL returns information about how it plans to execute the query. This information is presented in the form of a table with the various columns each providing insights into the different aspects of the query execution.

Examples 1: Total Orders by Customer

Consider a scenario where we have a table named orders with the columns order_id, customer_id, order_date and total_amount. We want to the retrieve the total amount of the orders placed by the each customer.

CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    customer_id INT,
    order_date DATE,
    total_amount DECIMAL(10, 2)
);

Sample Data:

INSERT INTO orders (order_id, customer_id, order_date, total_amount) VALUES
(1, 1, '2024-04-01', 100.00),
(2, 1, '2024-04-05', 150.00),
(3, 2, '2024-04-10', 200.00),
(4, 2, '2024-04-15', 120.00);

EXPLAIN SELECT customer_id, SUM(total_amount) AS total_orders 
FROM orders 
GROUP BY customer_id;

Output:

+----+-------------+--------+-------+--------------+------+---------+------+-----+-------+
| id | select_type | table  | type  | possible_keys| key  | key_len | ref  | rows| Extra |
+----+-------------+--------+-------+--------------+------+---------+------+-----+-------+
|  1 | SIMPLE      | orders | ALL   | NULL         | NULL | NULL    | NULL |   4 |       | Using where; Using filesort 
+----+-------------+--------+-------+--------------+------+---------+------+-----+-------+

Explanation:

• The output reveals that the query performs the simple SELECT operation on the orders table.
• It utilizes an index for the GROUP BY operation resulting in the efficient query execution.

Example 2: Product Listing by Category and Price

Let’s consider another scenario where we have a table named products with the columns product_id, category_id, name and price. We want to the retrieve all products from the specific category ordered by the price in the descending order.

Create the products table

CREATE TABLE products (
    product_id INT AUTO_INCREMENT PRIMARY KEY,
    category_id INT,
    name VARCHAR(255),
    price DECIMAL(10, 2)
);

Insert sample data into the products table

INSERT INTO products (category_id, name, price) VALUES
(1, 'Product 1', 10.99),
(1, 'Product 2', 20.49),
(2, 'Product 3', 15.99),
(1, 'Product 4', 8.50),
(3, 'Product 5', 30.00);

we can execute the EXPLAIN statement with the query:

EXPLAIN SELECT * 
FROM products 
WHERE category_id = 1 
ORDER BY price DESC;

+----+-------------+----------+------+---------------+------+---------+------+------+---------------------------------+
| id | select_type |  table   | type | possible_keys | key  | key_len | ref  | rows |             Extra               |
+----+-------------+----------+------+---------------+------+---------+------+------+---------------------------------+
|  1 |   SIMPLE    | products | ALL  |     NULL      | NULL |   NULL  | NULL |   5  | Using where; Using filesort     |
+----+-------------+----------+------+---------------+------+---------+------+------+---------------------------------+

Explanation:

• The output indicates a simple SELECT operation on the products table.
• It utilizes an index on category_id column for the efficient data retrieval.
• The ORDER BY clause requires a filesort operation due to the descending order by the price.

Understanding how to read MySQL EXPLAIN output is essential for the database developers and administrators to the optimize query performance. By analyzing the output they can identify potential performance bottlenecks such as the full table scans or inefficient index usage and take appropriate measures to the improve query execution efficiency.