Understanding explain in MySQL
As a MySQL developer, it's crucial to understand the performance of your queries. MySQL provides a tool called explain that can help you understand how your queries are being executed. In this post, we will dive deeper into explain and explore the various formats it offers.
To explain a query, you simply prefix your query with explain. MySQL will then output the query execution plan. Let's consider an example:
explain select * from people where first_name = "Aaron"
In the above query, MySQL will output information about the query plan, including any index used and an estimate of the number of rows being accessed.
Different formats of explain
The explain statement provides several formats that you can use to analyze your queries in more detail. Some of the commonly used explain formats are tree, JSON, and EXPLAIN ANALYZE.
Explain tree format
The tree format is useful for providing more detail into the execution plan in a nested tree structure. You can specify this format by adding format=tree at the beginning of the explain statement.
For example, consider the following query:
explain format=tree select * from people where first_name = "Aaron"
We're providing with the following output, which estimates the cost of each part of the query. (This query only has one part, so it doesn't look too much like a tree!)
-> Index lookup on people using multi (first_name='Aaron') (cost=198.00 rows=180)
Explain JSON format
The JSON format provides a more detailed view of the same information as provided in the tree format. You can specify this format by adding format=json at the beginning of the explain statement.
In the JSON format, MySQL provides information about the query's index usage, key parts, and query cost in a machine-readable, JSON format.
Explain analyze format
EXPLAIN ANALYZE actually runs the query and provides detailed statistics on the query's execution plan. It's important to note that this format actually runs the query, so it should be used with caution.
If you use the EXPLAIN ANALYZE format, MySQL will provide the detailed statistics that include the actual execution time and number of records read. If you're working with queries involving joins or subqueries, these statistics can be invaluable for understanding where the query is spending its time.
Conclusion
Explain in MySQL can significantly help optimize the performance of queries by providing insight into how they are executed. In this video, we discussed the different formats of explain that are available, including tree, JSON, and analyze formats.
While the various formats may seem overwhelming, it's essential to understand the output of the basic explain query. Even if you're new to MySQL, understanding the basic explain output will give you an advantage over most application developers.