Selected_related vs Prefetch_related

selected_related()

• "follows" foreign-key relationships and one to one relationships, selecting additional related-object data when it executes its query
• SQL로 풀어보면 Inner Join을 하여 관련 개체의 필드를 Select 문에 포함시켜서 관련 개체를 얻는다.
• SQL join get result backk as part of the table from SQL server
• SQL의 JOIN을 사용하는 특성상 foreign-key , one-to-one 와 같은 single-valued relationships에서만 사용이 가능하다는 한계가 있다.

Bad Example & Result

Good Example & Result

prefetch_related()

• does a seperate lookup for each realationship and does the "joining" in python (many to many)
• executes another query and reduce redundant columns create and send a list of id's to select back to the service
• 모델별 쿼리를 실행해 쿼리셋을 가져온다.
• foreign-key , one-to-one 뿐만 아니라 many-to-many , many-to-one 등 모든 relationships에서 사용 가능하다.

정참조 Good Example and result

역참조 Good Example and result

병민님 example:
-https://github.com/wecode-bootcamp-korea/queryset-api/tree/main/books

reference :

• https://medium.com/better-programming/django-select-related-and-prefetch-related-f23043fd635d
• https://jupiny.tistory.com/entry/selectrelated%EC%99%80-prefetchrelated
• https://bmh8993.github.io/Django/select_related%EC%99%80-prefetch_related%EB%A5%BC-%EC%A0%95%EB%B3%B5%ED%95%B4%EB%B3%B4%EC%9E%90/