休眠条件 OneToOne FetchType.Eager FetchMode.Join 执行不必要的查询

Question

I have sample entity TableA and TableB, in table A we have column TABLE_B_ID annotated as bellow:

@OneToOne(fetch = FetchType.EAGER, optional = false)
@JoinColumn(name = "TABLE_B_ID")
@Fetch(FetchMode.JOIN)

Then when i execute simple code like this:

Criteria c = session.createCriteria(TableA.class);
c.list();

Hibernate run query for n+1 times where n=number-of-row in TableA, even when TableA already joinned with TableB on the first query, Hibernate still trying to get TableB for each A by another select query.

Query fired:

Hibernate: 
select
    this_.id as id1_0_1_,
    this_.name as name2_0_1_,
    this_.TABLE_B_ID as TABLE_B_3_0_1_,
    tableb2_.id as id1_1_0_,
    tableb2_.name as name2_1_0_ 
from
    TABLE_A this_ 
inner join
    TABLE_B tableb2_ 
        on this_.TABLE_B_ID=tableb2_.id
Hibernate: 
    select
        tablea0_.id as id1_0_1_,
        tablea0_.name as name2_0_1_,
        tablea0_.TABLE_B_ID as TABLE_B_3_0_1_,
        tableb1_.id as id1_1_0_,
        tableb1_.name as name2_1_0_ 
    from
        TABLE_A tablea0_ 
    inner join
        TABLE_B tableb1_ 
            on tablea0_.TABLE_B_ID=tableb1_.id 
    where
        tablea0_.TABLE_B_ID=?
Hibernate: 
    select
        tablea0_.id as id1_0_1_,
        tablea0_.name as name2_0_1_,
        tablea0_.TABLE_B_ID as TABLE_B_3_0_1_,
        tableb1_.id as id1_1_0_,
        tableb1_.name as name2_1_0_ 
    from
        TABLE_A tablea0_ 
    inner join
        TABLE_B tableb1_ 
            on tablea0_.TABLE_B_ID=tableb1_.id 
    where
        tablea0_.TABLE_B_ID=?

and so on..

I already try to remove the FetchMode or change optional=true but the query fired is still n+1 times. What is the cause of hibernate firing another select even after he got the data on the first select query already?

Answer 1

You are experiencing a well known problem, the "N+1 selects" problem occurs when you select a parent entity and hibernate will make additional select for a child related to the parent with OneToOne. So if you have "N" parent-child records in the database, hibernate will get all parents with one select and then get each child in separated select, making total N+1 selects. There are two approaches for "N+1" problem in hibernate: 1. "Join Fetch" all OneToOne children. 2. Enable the second level cache and use @Cache annotation on the OneToOne children.

Your problem is that you didn't "join fetch" all of the OneToOne children. You must "join fetch" them all, including the transitive children (entities referenced from children themselves, or within the collection).

Making OneToOne lazy (because its eager by default) is only partial solution, because hibernate will make a select for a child only when you access some getter on the child, but in long term it will still make all the N selects.