Lära What is EXPLAIN? | Introduction to EXPLAIN and Query Plans

When working with SQL databases, understanding how queries are executed is essential for writing efficient and high-performing code. The EXPLAIN statement is a powerful tool that allows you to see the query plan the database engine creates for your SQL statements. By using EXPLAIN, you gain insight into how your queries interact with the underlying tables and indexes, helping you to identify potential bottlenecks or inefficiencies before they impact your application's performance.

The primary role of EXPLAIN is to display the steps the query planner will use to execute a given SQL statement. This includes information about which tables are accessed, in what order, what indexes (if any) are used, and how rows are filtered or joined. By examining this output, you can make informed decisions about rewriting queries, adding indexes, or restructuring your schema for better performance.


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-- See how EXPLAIN works with a simple SELECT on the products table
EXPLAIN SELECT * FROM products WHERE price > 50;

When you run an EXPLAIN statement, the output provides several columns that describe each step of the query plan. Some of the most important columns to focus on are:

id: shows the sequence of operations in the query plan;
select_type: indicates the type of SELECT operation, such as simple or a subquery;
table: displays the name of the table being accessed at each step;
type: reveals the join or access method used, such as ALL (full table scan), index, or ref.

Reading these columns helps you understand how the database processes your query and where improvements might be possible.


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-- Example: Using EXPLAIN on a JOIN between orders and customers
EXPLAIN
SELECT o.order_id, c.first_name, c.last_name, o.order_date
FROM orders o
JOIN customers c ON o.customer_id = c.customer_id;

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Avsnitt 1. Kapitel 1

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              12
            
-- See how EXPLAIN works with a simple SELECT on the products table
EXPLAIN SELECT * FROM products WHERE price > 50;

When you run an EXPLAIN statement, the output provides several columns that describe each step of the query plan. Some of the most important columns to focus on are:

id: shows the sequence of operations in the query plan;
select_type: indicates the type of SELECT operation, such as simple or a subquery;
table: displays the name of the table being accessed at each step;
type: reveals the join or access method used, such as ALL (full table scan), index, or ref.

Reading these columns helps you understand how the database processes your query and where improvements might be possible.


              12345
            
-- Example: Using EXPLAIN on a JOIN between orders and customers
EXPLAIN
SELECT o.order_id, c.first_name, c.last_name, o.order_date
FROM orders o
JOIN customers c ON o.customer_id = c.customer_id;

Var allt tydligt?

Tack för dina kommentarer!

Avsnitt 1. Kapitel 1

What is EXPLAIN?

1. What is the primary purpose of the EXPLAIN statement in SQL?

2. Which EXPLAIN output column indicates the table being accessed?

3. How does EXPLAIN help in query optimization?

What is EXPLAIN?

1. What is the primary purpose of the EXPLAIN statement in SQL?

2. Which EXPLAIN output column indicates the table being accessed?

3. How does EXPLAIN help in query optimization?