How can a query execution plan be used to optimize database performance?

The query execution plan shows how the DBMS will execute an SQL query: which indexes to use, in what order to join tables, what filtering and sorting operations to apply. By analyzing this plan, bottlenecks can be identified and the query can be optimized.

How to apply:

• Obtain the execution plan using the EXPLAIN command (or similar) in your DBMS.
• Check whether indexes are used or if a full table scan occurs.
• Verify the order of table joins and the number of rows processed at each step.
• Optimize the query or data structure: add indexes, rewrite the query, change JOINs, use aggregates and filters more effectively.

Example in PostgreSQL:

EXPLAIN ANALYZE SELECT * FROM orders WHERE customer_id = 123;

In Python, when working with a database, you can obtain and analyze the plan to understand how to improve the query and speed up the application.