EXPLAIN statement

Owned by Computer Science Department
Aug 02, 2023
2 min read

In SQL, the EXPLAIN statement (sometimes referred to as the EXPLAIN PLAN statement) is a powerful tool used to analyze and understand how the database management system (DBMS) executes a given SQL query. Its primary purpose is to provide insights into the query execution plan, which is the sequence of steps the DBMS follows to retrieve the requested data.

The EXPLAIN statement is not part of the standard SQL syntax and may have variations across different database management systems. However, it is widely supported in popular SQL databases like MySQL, PostgreSQL, Oracle, and others. Let's explore the basic usage and common features of the EXPLAIN statement:

Syntax:

EXPLAIN SELECT columns FROM table WHERE condition;

Usage:

  1. The EXPLAIN statement is used before an actual SELECT, UPDATE, DELETE, or INSERT query.

  2. It is essential to remember that the EXPLAIN statement does not execute the query or return the actual query results. Instead, it provides details about how the query would be executed.

Output:
The output of the EXPLAIN statement typically includes information such as:

  • The order in which tables are accessed (in multi-table queries)

  • The type of join used (if applicable)

  • The indexes used, if any

  • The estimated number of rows examined or returned by each step

  • The cost and performance-related statistics

Interpreting the Output:

  • The output helps identify potential performance bottlenecks and whether the query is using indexes efficiently.

  • It allows database administrators and developers to optimize query execution by making informed decisions, such as creating or modifying indexes, rewriting queries, or adjusting configuration settings.

Example:
Let's consider a simple example using the MySQL database:

EXPLAIN SELECT first_name, last_name FROM employees WHERE department = 'IT';

The output might look something like this:

+----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | employees| ref | department | department| 10 | const| 100 | Using index | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+

In this example, the output indicates that the DBMS uses the "department" index efficiently (key = department) and that it expects to examine 100 rows (rows = 100) to fulfill the query.

Keep in mind that the EXPLAIN statement provides estimates based on statistics, and the actual execution plan might differ based on the data distribution and other factors. Nonetheless, it is an indispensable tool for query optimization and performance tuning in SQL databases.