Merge n+1 problem

Hibernate ORM
1

Hi experts!

I have a spring boot aplication which has a biderectional one-to-many mapping with both sides lazy. When data comes from presentation layer I do a merge operation through a parent entity. It’s an edit operation so parent and child entities all have ids.
But it seems that there is so called “n+1” problem. I can see in logs that hibernate does an additional select to load each child entity. It doesn’t make a join to load all data at once.

In one of my prevous projects we used another jpa provider(not hibernate) and had a similliar issue, so a “hacky” solution was to preload data in persistent context mannualy using one query before calling merge.

Do I miss something in project setting or entity mappings? I think there shouldn’t be such an issue with hibernate.

2

Hi,
You should load the modified entities before copying your DTOs into these entities. Use left join fetch to load all of them efficiently.
…do not use merge here

3

If you want to fix/improve this in Hibernate, you can create a JIRA issue and discuss your idea for a solution with us on Zulip. It’s a tradeoff as you can imagine. Join fetching too much can lead to performance issues as well and might even be unnecessary if there are many “null” FKs, but I think it would be great if we would introduce an option to configure the behavior.