What does the EXPLAIN command show and what parameters does it output for SQL query performance analysis?

The EXPLAIN command in SQL is used to analyze the execution plan of a query. It shows how the database management system (DBMS) will execute the query, including which indexes and data access methods will be used.

Main parameters displayed by EXPLAIN:

• id — the step identifier, helps understand the order of operations.
• select_type — the type of SELECT query (simple, subquery, etc.).
• table — the table to which the operation applies.
• type — the join or access type (e.g., ALL — full table scan, index, ref, const, etc.). The better the type, the more efficient.
• possible_keys — indexes that could be used.
• key — the index actually used.
• key_len — the length of the used key.
• ref — columns or constants compared with the index.
• rows — estimated number of rows to be processed at this step.
• Extra — additional information, such as use of temporary tables, sorting, filters.

Example of EXPLAIN output:

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	users	ref	idx_name	idx_name	102	const	10	Using where

This helps optimize queries, for example, by adding indexes or rewriting queries to reduce the number of rows processed.