As soon as I increased the data to hundreds of thousands of records, the server died and became slow just from multiple users querying it, and now I think I'm starting to understand why. There were many cases of performance optimization by applying indexes, so I applied them that way, but I hadn't even verified whether I was actually using them correctly in the past, which makes me regret it. Through this lecture, I learned that even good features have drawbacks and must be applied according to the service, and that it's not simply about improving performance, but how important it is to understand and design correctly. I learned a lot thanks to the good lecture, and I'm really looking forward to the design and optimization lectures too. Especially the design lecture!! I want to expand my understanding of databases even deeper.