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EXPLAIN access types

Understanding MySQL's explain types

The explain function allows you to see how MySQL executes a query, providing valuable insights into how your database is functioning. In this article, we'll take a deep dive into one of the most important columns - type.


At the top of the list, the const access method is one of the most efficient. Const access is only used when a primary key or unique index is in place, allowing MySQL to locate the necessary row with a single operation. When you see const in the type column, it's telling you that MySQL knows there is only one match for this query, making the operation as efficient as possible.


The ref access method is slightly less efficient than const, but still an excellent choice if the right index is in place. Ref access is used when the query includes an indexed column that is being matched by an equality operator. If MySQL can locate the necessary rows based on the index, it can avoid scanning the entire table, speeding up the query considerably.


MySQL provides an option to create full-text indexes on columns intended for text-based search queries. The fulltext access method is used when a full-text index is in place and the query includes a full-text search. Fulltext access allows MySQL to search the index and return the results quickly.


When you use range in the where clause, MySQL knows that it will need to look through a range of values to find the right data. MySQL will use the B-Tree index to traverse from the top of the tree down to the first value of the range. From there, MySQL consults the linked list at the bottom of the tree to find the rows with values in the desired range. It's essential to note that MySQL will examine every element in the range until a mismatch is found, so this can be slower than some of the other methods mentioned so far.


The index access method indicates that MySQL is scanning the entire index to locate the necessary data. Index access is the slowest access method listed so far, but it is still faster than scanning the entire table. When MySQL cannot use a primary or unique index, it will use index access if an index is available.


Finally, the all access method means that MySQL is scanning the entire table to locate the necessary data. All is the slowest and least efficient access method, so it's one that you want to avoid as much as possible. MySQL may choose to scan the entire table when there is no suitable index, so this is an excellent opportunity to audit your indexing strategy.

Improving query performance

Using explain types, you can gain insight into your query performance and identify areas for improvement. If you see const or ref access methods, you likely have a well-structured database that performs well. If you see index or all access methods, it might be time to investigate ways to optimize your database structure or indexing strategy.

By taking a proactive approach to query optimization, you can boost database performance and provide a better end-user experience. The type column is just one tool that you can use to gain insights into your data and improve query performance. Try experimenting with different types of queries and see how MySQL approaches them, using the type column to gauge the efficiency of your database's access methods.

About this lesson

In this video we dive deep into the `type` column from the EXPLAIN output. This is one of the most important pieces of information that the EXPLAIN statement gives you.

Closed captioned

Meet your instructor, Aaron Francis

Developer Education

I’m a dad to one-year-old twins and a software developer with a keen interest in processes and automation.

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