JPA eager fetch does not join

by

JPA doesn’t provide any specification on mapping annotations to select fetch strategy. In general, related entities can be fetched in any one of the ways given below

  • SELECT => one query for root entities + one query for related mapped entity/collection of each root entity = (n+1) queries
  • SUBSELECT => one query for root entities + second query for related mapped entity/collection of all root entities retrieved in first query = 2 queries
  • JOIN => one query to fetch both root entities and all of their mapped entity/collection = 1 query

So SELECT and JOIN are two extremes and SUBSELECT falls in between. One can choose suitable strategy based on her/his domain model.

By default SELECT is used by both JPA/EclipseLink and Hibernate. This can be overridden by using:

@Fetch(FetchMode.JOIN) 
@Fetch(FetchMode.SUBSELECT)

in Hibernate. It also allows to set SELECT mode explicitly using @Fetch(FetchMode.SELECT) which can be tuned by using batch size e.g. @BatchSize(size=10).

Corresponding annotations in EclipseLink are:

@JoinFetch
@BatchFetch

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