How can i resolve the N+1 Selects problem?

Question

I have trouble understanding how to avoid the n+1 select in jpa or hibernate.

From what i read, there's the 'left join fetch', but i'm not sure if it still works with more than one list (oneToMany)..

Could someone explain it to me, or give me a link with a clear complete explanation please ?

I'm sorry if this is a noob question, but i can't find a real clear article or doc on this issue.

Thanks

Answer 1

Apart from the join, you can also use subselect(s). This results in 2 queries being executed (or in general m + 1, if you have m lists), but it scales well for a large number of lists too, unlike join fetching.

With join fetching, if you fetch 2 tables (or lists) with your entity, you get a cartesian product , ie all combinations of pairs of rows from the two tables. If the tables are large, the result can be huge , eg if both tables have 1000 rows, the cartesian product contains 1 million rows!

A better alternative for such cases is to use subselects. In this case, you would issue 2 selects - one for each table - on top of the main select (which loads the parent entity), so altogether you load 1 + 100 + 100 rows with 3 queries.

For the record, the same with lazy loading would result in 201 separate selects, each loading a single row.

Update: here are some examples:

• a tutorial: Tuning Lazy Fetching , with a section on subselects towards the end (btw it also explains the n+1 selects problem and all strategies to deal with it),
• examples of HQL subqueries from the Hibernate reference,
• just in case, the chapter on fetching strategies from the Hibernate reference - similar content as the first one, but much more thorough