How To NOT Use Spring Data Streamable

Description: This application is a sample of fetching Streamable<entity> and Streamable<dto>. But, more important, this application contains three examples of how to not use Streamable. It is very tempting and comfortable to fetch a Streamable result set and chop it via filter(), map(), flatMap(), and so on until we obtain only the needed data instead of writing a query (e.g., JPQL) that fetches exactly the needed result set from the database. Mainly, we just throw away some of the fetched data to keep only the needed data. But, is not advisable to follow such practices because fetching more data than needed can cause significant performance penalties.

Moreover, pay attention to combining two or more Streamable via the and() method. The returned result may be different from what you are expecting to see. Each Streamable produces a separate SQL statement and the final result set is a concatenation of the intermediate results sets (prone to duplicate values).

Key points:

• don't fetch more columns than needed just to drop a part of them (e.g., via map())
• don't fetch more rows than needed just to throw away a part of it (e.g., via filter())
• pay attention on combining Streamable via and(); each Streamable produces a separate SQL statement and the final result set is a concatenation of the intermediate results sets (prone to duplicate values)

---

If you need a deep dive into the performance recipes exposed in this repository then I am sure that you will love my book "Spring Boot Persistence Best Practices"	If you need a hand of tips and illustrations of 100+ Java persistence performance issues then "Java Persistence Performance Illustrated Guide" is for you.

---