title: Hibernate Subselect vs Batch Fetching
body: <p>Hibernate provides (at least) two options for getting around the N+1 query problem. The one is setting the FetchMode to Subselect, which generates a select with a IN-clause and a subselect within this IN-clause. The other is to specify a BatchSize, which generates a select with a IN-clause containing the parents' IDs.</p>
  
  <p>Both work but I find that the Subselect option often runs into performance problems due to the query for the parents being complex. On the other hand, with a large BatchSize (say 1000), the number of queries and complexity of those queries are very small.</p>
  
  <p>My question is thus: when would you use Hibernate's Subselect FetchMode over BatchSize? Subselect probably makes sense if you have a very large number of parent entries (thousands), but are there any other scenarios where you'd prefer a Subselect to BatchSize?</p>
  
  <p>EDIT: I noticed a difference between the two when dealing with eager loading. If you have an xToMany association set to be loaded eagerly and through a subselect, it generates a subselect like it would if it was lazy. If you specify a BatchSize however, the generated query  makes use of a outer join instead of a seperate query. Is there any way to force Hibernate to use a seperate batched query when loading eagerly?</p>