- Why do we want join fetch? JPA default fetch strategy is "Lazy", which has better performance under most circumstances. But, sometimes we do want to retrieve all children (and sometimes children's children). In that case, join fetch allows us to get everything in one Database round-trip instead of N (or N*N round-tips). This change can easily speed up those queries for 100 times (in my case it went from 20 seconds to 0.2 seconds).
- Make sure in your Entity classes, the child collections are "Set
" not "List ". If you get the error message: "Hibernate cannot simultaneously fetch multiple bags". This is the root cause.
- How to join multiple levels:
root .fetch([childAttributeName], JoinType.LEFT) .fetch([grandChildAttributeName], JoinType.LEFT);
- Fetch join will return multiple duplicated rows for the parent entity. This is usually undesirable, wrap the return set in a LinkedHashSet will get the unique parent entities in the original select order. (See this Stack Overflow post)