我正在使用Spring Data JPA作为持久层，但是我遇到了N+1问题。同时我也在使用Specifications API，因此我发现很难解决N+1问题。请帮助。
@Entity
public class PopulationHealth {
@Id
private int caseId;
@OneToMany(mappedBy = "caseId", fetch = FetchType.LAZY)
private List<CostSaving> costSavings;
}
public class CostSaving {
@Id
private int caseId;
}
@Transactional(readOnly = true)
public interface PopulationHealthRepository extends JpaRepository<PopulationHealth, Integer>, JpaSpecificationExecutor<PopulationHealth> {
Page<PopulationHealth> findAll(Specification<PopulationHealth> spec, Pageable pageable);
}
在下面的类中，我后来添加了root.join()方法，这将在查询中创建左连接，但N+1问题仍然存在。请参考以下日志输出：
public class PopulationHealthSearchSpec implements Specification<PopulationHealth> {
private List<PopulationHealthCriteriaDto> criteria;
public PopulationHealthSearchSpec() {
criteria = new ArrayList<>();
}
public void addCriteria(List<PopulationHealthCriteriaDto> criteria) {
this.criteria.addAll(criteria);
}
@Override
public Predicate toPredicate(Root<PopulationHealth> root, CriteriaQuery<?> query, CriteriaBuilder cb) {
List<Predicate> predicates = new ArrayList<>();
criteria.forEach(p -> {
SearchOperation operation = p.getOperation();
root.join("costSavings", JoinType.LEFT);
switch (operation) {
case GREATER_THAN:
predicates.add(cb.greaterThan(root.get(p.getKey().toString()), convertToDate(p.getValue())));
break;
case GREATER_THAN_EQUAL:
predicates.add(cb.greaterThanOrEqualTo(root.get(p.getKey().toString()), convertToDate(p.getValue())));
break;
case LESS_THAN:
predicates.add(cb.lessThan(root.get(p.getKey().toString()), convertToDate(p.getValue())));
break;
case LESS_THAN_EQUAL:
predicates.add(cb.lessThanOrEqualTo(root.get(p.getKey().toString()), convertToDate(p.getValue())));
break;
case EQUAL:
predicates.add(cb.equal(root.get(p.getKey().toString()), p.getValue()));
break;
case NOT_EQUAL:
predicates.add(cb.notEqual(root.get(p.getKey().toString()), p.getValue()));
break;
case IN:
predicates.add(getInPredicates(cb, root, p));
break;
case NOT_IN:
predicates.add(getInPredicates(cb, root, p).not());
break;
}
});
return cb.and(predicates.toArray(new Predicate[0]));
}
使用N+1查询记录输出。第一次查询使用了左连接，但仍存在N+1问题。
2020-05-06 19:46:41,527 DEBUG org.hibernate.SQL : select population0_.CaseId as CaseId1_3_, population0_.CaseCreateDate as CaseCrea2_3_ from POPULATION_HEALTH_UNMASKED population0_ left outer join COST_SAVINGS costsaving1_ on population0_.CaseId=costsaving1_.CaseId where population0_.CaseId in (3098584 , 3098587 , 3098591) order by population0_.CaseCreateDate asc limit ?
2020-05-06 19:46:41,709 DEBUG org.hibernate.SQL : select costsaving0_.CaseId as CaseId1_1_0_, costsaving0_.CaseId as CaseId1_1_1_ from COST_SAVINGS costsaving0_ where costsaving0_.CaseId=?
2020-05-06 19:46:41,744 DEBUG org.hibernate.SQL : select costsaving0_.CaseId as CaseId1_1_0_, costsaving0_.CaseId as CaseId1_1_1_ from COST_SAVINGS costsaving0_ where costsaving0_.CaseId=?
2020-05-06 19:46:41,781 DEBUG org.hibernate.SQL : select costsaving0_.CaseId as CaseId1_1_0_, costsaving0_.CaseId as CaseId1_1_1_ from COST_SAVINGS costsaving0_ where costsaving0_.CaseId=?
在Specification类中使用fetch()而不是join()后，我遇到了以下问题：
2020-05-06 20:29:25,315 ERROR org.apache.catalina.core.ContainerBase.[Tomcat].[localhost].[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.dao.InvalidDataAccessApiUsageException: org.hibernate.QueryException: query specified join fetching, but the owner of the fetched association was not present in the select list [FromElement{explicit,not a collection join,fetch join,fetch non-lazy properties,classAlias=generatedAlias1,role=com.cambia.mmt.cdqs.api.entity.PopulationHealth.costSavings,tableName=COST_SAVINGS,tableAlias=costsaving1_,origin=POPULATION_HEALTH_UNMASKED population0_,columns={population0_.CaseId ,className=com.cambia.mmt.cdqs.api.entity.CostSaving}}] [select count(generatedAlias0) from com.cambia.mmt.cdqs.api.entity.PopulationHealth as generatedAlias0 left join fetch generatedAlias0.costSavings as generatedAlias1 where generatedAlias0.caseCreateDate>:param0]; nested exception is java.lang.IllegalArgumentException: org.hibernate.QueryException: query specified join fetching, but the owner of the fetched association was not present in the select list [FromElement{explicit,not a collection join,fetch join,fetch non-lazy properties,classAlias=generatedAlias1,role=com.cambia.mmt.cdqs.api.entity.PopulationHealth.costSavings,tableName=COST_SAVINGS,tableAlias=costsaving1_,origin=POPULATION_HEALTH_UNMASKED population0_,columns={population0_.CaseId ,className=com.cambia.mmt.cdqs.api.entity.CostSaving}}] [select count(generatedAlias0) from com.cambia.mmt.cdqs.api.entity.PopulationHealth as generatedAlias0 left join fetch generatedAlias0.costSavings as generatedAlias1 where generatedAlias0.caseCreateDate>:param0]] with root cause
org.hibernate.QueryException: query specified join fetching, but the owner of the fetched association was not present in the select list [FromElement{explicit,not a collection join,fetch join,fetch non-lazy properties,classAlias=generatedAlias1,role=com.cambia.mmt.cdqs.api.entity.PopulationHealth.costSavings,tableName=COST_SAVINGS,tableAlias=costsaving1_,origin=POPULATION_HEALTH_UNMASKED population0_,columns={population0_.CaseId ,className=com.cambia.mmt.cdqs.api.entity.CostSaving}}]
fetch而不是join，因为root对您可用。我认为还有其他东西正在处理结果（例如结果序列化？），这会导致关系被初始化。 - Antoniossss