What does the EXPLAIN command show, what data does it output?

The EXPLAIN command is used in SQL to analyze the execution plan of a query by the database. It shows how the DBMS intends to execute the query: which indexes will be used, the order of table scans, types of joins, and estimates of the number of rows at each stage.

The data provided by EXPLAIN helps understand how efficiently the query is written and where it can be optimized.

Example output of EXPLAIN (PostgreSQL):

EXPLAIN SELECT * FROM users WHERE id = 10;

The result may include:

• Type of scan (Seq Scan, Index Scan)
• Cost estimate
• Row count estimate
• Used indexes

This allows developers or DBAs to improve the query, for example, by adding an index or rewriting it.