Mastering MariaDB Query Optimization: EXPLAIN in the Slow Query Log

Unlocking MariaDB Query Performance: EXPLAIN and Slow Query Log


Understanding how queries are executed by a database system is paramount for database administrators and developers seeking to optimize performance. In the realm of MariaDB, the EXPLAIN statement shines as a crucial tool to dissect and analyze query execution plans. When combined with the slow query log, it becomes a powerful means to identify bottlenecks and enhance the efficiency of slower queries.

In this guide, we delve into the intricacies of using EXPLAIN in the slow query log in MariaDB. We’ll uncover how to set up the slow query log, decipher the output of the EXPLAIN statement, and employ practical real-life examples to shed light on its application in query optimization.

What is EXPLAIN?

The EXPLAIN statement in MariaDB provides information about how a query is executed by the database engine. It reveals details such as the order in which tables are accessed, the type of join used, and whether indexes are utilized. By examining the query execution plan, you can pinpoint areas for optimization.

Setting Up the Slow Query Log

Before using EXPLAIN in conjunction with the slow query log, you need to ensure that the slow query log is enabled and properly configured in your MariaDB server’s configuration file (usually my.cnf or my.ini). Here’s a basic configuration example:

slow_query_log = 1
slow_query_log_file = /var/log/mysql/slow-query.log
long_query_time = 1

In this example, we enable the slow query log, specify the log file’s location, and set the threshold (long_query_time) for queries considered “slow” to 1 second.

Analyzing Slow Queries with EXPLAIN

To analyze slow queries using EXPLAIN, follow these steps:

  • Identify the slow query in the slow query log file. You can use tools like grep to search for queries that exceed the long_query_time threshold.
  • Copy the slow query (excluding the EXPLAIN keyword) to the MariaDB command-line interface or a database client.
  • Prefix the query with EXPLAIN and execute it.

Practical Examples with a Real-Life Dataset

Let’s consider a real-life scenario where we have an e-commerce database with two tables: orders and order_items. We want to analyze a slow query that retrieves the total revenue for a specific date range. Here’s the slow query:

SELECT SUM(order_items.price * order_items.quantity) AS revenue
FROM orders
JOIN order_items ON orders.order_id = order_items.order_id
WHERE orders.order_date BETWEEN ‘2023-01-01’ AND ‘2023-02-28’;

To analyze this query, we would:

  • Identify it in the slow query log.
  • Copy the query to the MariaDB client.
  • Execute EXPLAIN with the query: EXPLAIN SELECT … (excluding the EXPLAIN keyword).

Interpreting EXPLAIN Output

The EXPLAIN output provides essential information about query execution. Key columns in the output include:

  • id: Represents the query block or subquery.
  • select_type: Describes the type of SELECT statement.
  • table: The table referenced in the row.
  • type: Indicates the access method (e.g., ALL, index, range).
  • possible_keys: Lists potential indexes that could be used.
  • key: The index selected for query optimization.
  • rows: The number of rows examined.
  • Extra: Additional information about the query execution.

By analyzing this output, you can identify potential issues like full table scans, inefficient joins, or missing indexes.

Query Optimization Techniques

Once you’ve identified performance issues using EXPLAIN, you can employ various optimization techniques, such as:

  • Indexing: Create appropriate indexes on columns used in WHERE clauses and join conditions.
  • Query Rewriting: Rewrite queries to use more efficient approaches.
  • Caching: Implement query result caching to reduce database load.
  • Table Partitioning: Partition large tables to improve query performance.
  • Hardware Scaling: Upgrade hardware or use sharding for scaling.


The fusion of EXPLAIN and the slow query log in MariaDB equips administrators and developers with a formidable toolkit for deciphering and optimizing query performance. By scrutinizing query execution plans, pinpointing potential issues, and implementing optimization techniques, you can elevate the efficiency and responsiveness of your MariaDB database.

Remember that the key to maintaining a high-performing database lies in continuous monitoring and proactive optimization. Regularly reviewing the slow query log, interpreting EXPLAIN output, and fine-tuning your queries are essential practices for ensuring that your database performs optimally, providing the best possible experience for your users and applications.

About Shiv Iyer 452 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.