How to Resolve Hibernate's MultipleBagFetchException When Fetching Multiple Collections?

Hibernate Encounters MultipleBagFetchException: Handling Simultaneous Queries for Multiple Collections

In an attempt to retrieve data from multiple collections simultaneously, programmers may encounter the exception "org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags." To address this issue, it's crucial to understand the underlying problem.

In the provided example, a Parent entity has a collection of Child entities, and another "grandparent" entity, AnotherParent, contains a collection of AnotherChild entities. By default, Hibernate eagerly fetches both collections at once. However, this approach can lead to a Cartesian product problem, resulting in an explosion of results.

To avoid this issue, there are two primary solutions:

• Use @LazyCollection Annotation: Annotate the collection fields with "@LazyCollection(LazyCollectionOption.FALSE)" and remove the "fetchType" attribute from the "@*ToMany" annotation. This instructs Hibernate to lazily load the collections only when they are needed, preventing the Cartesian product problem.
• Consider Using a Set: In most cases, using a Set is more suitable than a List. Sets inherently prevent duplicate elements, eliminating the possibility of null children in the list.

Cautionary Note:

While using a Set may not eliminate the underlying Cartesian product issue, it does reduce the impact by eliminating the potential for duplicate elements. However, for significant datasets, even a Set can encounter performance issues due to the underlying join operation.

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