How to use Explain in MySQL for Query Optimization

When it comes to MySQL query optimization, EXPLAIN is your trusted ally. This tool provides a clear view into how MySQL executes your queries, helping you identify and rectify performance bottlenecks. In this concise guide, we’ll show you how to use EXPLAIN effectively and improve your database queries.

Understanding EXPLAIN Output

When you run an EXPLAIN statement, you receive a result set with valuable information about the query’s execution plan. Key columns to focus on include:

  • type: Describes how MySQL retrieves rows from the table. “ALL” is generally slow, while “index” is faster.
  • key: Shows which index is used, or “NULL” if none is.
  • rows: Estimates the number of rows examined, with fewer rows being better for performance.

Using EXPLAIN for Optimization

  1. Identify Slow Queries: Start by finding the queries causing performance issues, usually by user complaints or monitoring system metrics.
  2. Interpret EXPLAIN Output: Analyze the output to identify potential problems, focusing on “type,” “key,” and “rows.”
  3. Optimize Indexes: If EXPLAIN reveals that no index is used, add a relevant index or revise existing ones.
  4. Refine SQL Queries: Based on EXPLAIN output, consider rewriting queries, adding conditions, or optimizing JOIN operations.
  5. Test and Benchmark: Always test the impact of your changes and benchmark query performance.

Real-world EXPLAIN Example

Imagine you have this simple query:

EXPLAIN SELECT * FROM products WHERE category = 'Electronics' AND price < 500;

Here’s the simplified EXPLAIN output:

 id| select_type| table| type | key | key_len  | ref  | rows| Extra|
- - - - - - - - - - - - - - - - - - - - - -- - - - - - -- - - - -
1 | SIMPLE | products|range |category| 13 | NULL| 1000| Using where

In this example, the type is “range,” indicating index usage. The key is “category,” showing index usage. However, the rows are 1000, which might be optimized further.


Mastering EXPLAIN is a fundamental skill for improving MySQL query performance. With its insights, you can spot slow queries, refine SQL statements, optimize indexes, and enhance your database-driven applications. As you become proficient, you’ll unlock the full potential of EXPLAIN for boosting query efficiency.

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