In the performance optimization explanation, instead of saying, "This is how it will be optimized," it explained where the fundamental problem is, how the problem occurred, and the result itself, saying, "As a result, it will be optimized like this." So I learned how to approach and solve problems I encounter during the API development process or while using JPA, and in the end, I learned that the core is relational DB and that JPA is just a tool that makes it easier to use relational DB. I got goosebumps throughout the lecture. (I was just amazed by the field know-how...) And it was so much fun. Thank you.