春數據JPA + QueryDSL查詢優化

Question

我所遇到的一個奇怪的行爲，當使用QueryDSL集成與Spring數據JPA：

我有項目和人之間的關係多對一。如果我得到所有者ID的所有屬於用戶項目（外鍵）一切正常：

QProject project = QProject.project; 
QPerson owner = project.owner; 
List<Project> projects = from(project).leftJoin(owner).fetch() 
    .where(owner.id.eq(id)).list(project); 

生成的查詢：

select 
    project0_.id as id1_1_0_, 
    person1_.id as id1_0_1_, 
    project0_.creation_date as creation2_1_0_, 
    project0_.name as name3_1_0_, 
    project0_.owner as owner4_1_0_, 
    person1_.name as name2_0_1_ 
from 
    project project0_ 
left outer join 
    person person1_ 
     on project0_.owner=person1_.id 
where 
    project0_.owner=? 

然而，假設我們希望獲得所有項目由字段不是外鍵（例如所有者的名稱）屬於一個人：

QProject project = QProject.project; 
QPerson owner = project.owner; 
List<Project> projects = from(project).leftJoin(owner).fetch() 
    .where(owner.name.eq(name)).list(project); 

在這種情況下，該表人是加入不必要的兩倍（通知person1_和person2_）：

select 
    project0_.id as id1_1_0_, 
    person1_.id as id1_0_1_, 
    project0_.creation_date as creation2_1_0_, 
    project0_.name as name3_1_0_, 
    project0_.owner as owner4_1_0_, 
    person1_.name as name2_0_1_ 
from 
    project project0_ 
left outer join 
    person person1_ 
     on project0_.owner=person1_.id cross 
join 
    person person2_ 
where 
    project0_.owner=person2_.id 
    and person2_.name=? 

任何想法爲什麼會發生這種情況，以及如何避免它？

Answer 1

您需要創建一個別名，以確保在其中部分

QProject project = QProject.project; 
QPerson owner = new QPerson("owner"); 
List<Project> projects = from(project) 
    .leftJoin(project.owner, owner).fetch() 
    .where(owner.name.eq(name)) 
    .list(project);