为什么 hibernate @ManyToOne 为同一个表生成多个 select 查询？

Question

Why Hibernate generate multiple select statement for same table/entity? Below is @ManyToOne Mapping:

class Student{

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "student_id", insertable = false, updatable = false, referencedColumnName = "student_id")
    Subject subject;
}

Generated HQL:3 times select query is genereating for subject table

Hibernate: 
    select
        subject0_.subject_cd as subject_cd1_12_0_,
        subject0_.subjectname as subject_name_12_0_ 
    from
        smart.vw_subject subject0_
    where
        fundhierar0_.subject_cd =?
Hibernate: 
    select
        subject0_.subject_cd as subject_cd1_12_0_,
        subject0_.subjectname as subject_name_12_0_ 
    from
        smart.vw_subject subject0_
    where
        fundhierar0_.subject_cd =?
Hibernate: 
    select
        subject0_.subject_cd as subject_cd1_12_0_,
        subject0_.subjectname as subject_name_12_0_ 
    from
        smart.vw_subject subject0_
    where
        fundhierar0_.subject_cd =?

Answer 1

Hibernate provides multiple strategies for retrieving data. Few are Select , join , batch .

However, this seems like N+1 problem. The first query retrieves N records from the database. For each Parent, a new query retrieves the Child. Therefore for N Parent, N queries retrieve information from the child table.

The fetching strategy depends on how our application works, so gonna leave you to that. You can find more details here