我有以下 jpa 标准查询:
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Company> cq = cb.createQuery(Company.class);
Root<Company> root = cq.from(Company.class);
cq.select(root);
Subquery<String> sq = cq.subquery(String.class);
Root<Employee> subroot = sq.from(Employee.class);
sq.select(subroot.get(Employee_.lastName));
Predicate typePredicate = cb.equal(subroot.get(Employee_.lastName), "Doe");
Predicate correlatePredicate = cb.equal(root.get(Company_.employees), subroot);
sq.where(cb.and(typePredicate, correlatePredicate));
cq.where(cb.exists(sq));
TypedQuery<Company> typedQuery = em.createQuery(cq);
List<Company> companies = typedQuery.getResultList();
Eclipselink 生成以下 SQL:
SELECT t0.ID, ... FROM COMPANY t0
WHERE EXISTS (SELECT t1.LASTNAME FROM EMPLOYEES t2, EMPLOYEES t1
WHERE (((t1.LASTNAME = ?) AND (t1.ID = t2.ID))
AND (t2.COMPANY_ID = t0.ID)))
正如您所看到的,表 EMPLOYEES 上存在不必要的联接。我如何摆脱这个连接?
最佳答案
您似乎不需要子查询来进行查询,只需一个联接就足够了,
http://en.wikibooks.org/wiki/Java_Persistence/Criteria#Join
否则,您使用的是什么版本?您可以尝试一下 EclipseLink 2.4吗?
如果仍然有重复项,请记录错误并为其投票。
您也许可以使用逆ManyToOne,而不是OneToMany(即 root == subroot.get("company") )。
也尝试一下2.4中的JPQL,连接优化了吗?
关于java - jpa子查询中不必要的连接,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/13704016/