如何在加載Hibernate實體時優化Carstesian產品

Question

爲了演示我的問題，我創建了一個簡單的Spring Boot應用程序。它有一個實體，它有ID，兩個String屬性和兩個Sets<String>集合。

package com.mk.cat.domain; 

import javax.persistence.*; 
import java.util.Set; 

@Entity 
@Table(name = "cat") 
public class Cat { 
    @Id 
    @GeneratedValue(strategy = GenerationType.IDENTITY) 
    @Column(name = "id") 
    private Long id; 

    @Column(name = "name") 
    private String name; 

    @Column(name = "sex") 
    private String sex; 

    @ElementCollection(fetch = FetchType.EAGER) 
    @Column(name = "color") 
    @CollectionTable(
      name = "cat_color", 
      joinColumns = @JoinColumn(name = "cat_id")) 
    private Set<String> colors; 

    @ElementCollection(fetch = FetchType.EAGER) 
    @Column(name = "nickname") 
    @CollectionTable(
      name = "cat_nickname", 
      joinColumns = @JoinColumn(name = "cat_id")) 
    private Set<String> nicknames; 

    public Long getId() { 
     return id; 
    } 

    public void setId(Long id) { 
     this.id = id; 
    } 

    public String getName() { 
     return name; 
    } 

    public void setName(String name) { 
     this.name = name; 
    } 

    public String getSex() { 
     return sex; 
    } 

    public void setSex(String sex) { 
     this.sex = sex; 
    } 

    public Set<String> getColors() { 
     return colors; 
    } 

    public void setColors(Set<String> colors) { 
     this.colors = colors; 
    } 

    public Set<String> getNicknames() { 
     return nicknames; 
    } 

    public void setNicknames(Set<String> nicknames) { 
     this.nicknames = nicknames; 
    } 
} 

還有一個簡單的代碼，它從數據庫中持久存儲並加載Cat實體。

package com.mk.cat; 

import com.google.common.collect.Sets; 
import com.mk.cat.domain.Cat; 
import org.slf4j.Logger; 
import org.slf4j.LoggerFactory; 
import org.springframework.beans.factory.annotation.Autowired; 
import org.springframework.boot.CommandLineRunner; 
import org.springframework.boot.SpringApplication; 
import org.springframework.boot.autoconfigure.SpringBootApplication; 

@SpringBootApplication 
public class CatApplication implements CommandLineRunner { 

    private final CatRepository catRepository; 

    private static final Logger LOGGER = LoggerFactory.getLogger(CatApplication.class); 

    @Autowired 
    public CatApplication(CatRepository catRepository) { 
     this.catRepository = catRepository; 
    } 

    public static void main(String[] args) { 
     SpringApplication.run(CatApplication.class, args); 
    } 

    @Override 
    public void run(String... args) throws Exception { 
     Cat cat = new Cat(); 
     cat.setName("Ben"); 
     cat.setSex("Male"); 
     cat.setNicknames(Sets.newHashSet("Fluffy", "Mr. Tomcat", "Catburger")); 
     cat.setColors(Sets.newHashSet("Black", "White")); 

     final Cat saved = catRepository.save(cat); 
     LOGGER.info("Cat saved={}", cat); 

     catRepository.findOne(saved.getId()); 
    } 
} 

我跟蹤Hibernate和我發現，該Cat從DB通過這個SQL加載。

select cat0_.id as id1_0_0_, 
    cat0_.name as name2_0_0_, 
    cat0_.sex as sex3_0_0_, 
    colors1_.cat_id as cat_id1_1_1_, 
    colors1_.color as color2_1_1_, 
    nicknames2_.cat_id as cat_id1_2_2_, 
    nicknames2_.nickname as nickname2_2_2_ 
    from cat cat0_ 
    left outer join cat_color colors1_ on cat0_.id=colors1_.cat_id 
    left outer join cat_nickname nicknames2_ on cat0_.id=nicknames2_.cat_id 
where cat0_.id=1 

的休眠然後獲取從cat表和兩個表中的行，表示所述Cat#colors和Cat#nicknames設置該笛卡兒積。

id1_0_0_ name2_0_0_ sex3_0_0_ cat_id1_1_1_ color2_1_1_ cat_id1_2_2_ nickname2_2_2_ 
1 Ben Male 1 Black 1 Fluffy 
1 Ben Male 1 Black 1 Catburger 
1 Ben Male 1 Black 1 Mr. Tomcat 
1 Ben Male 1 White 1 Fluffy 
1 Ben Male 1 White 1 Catburger 
1 Ben Male 1 White 1 Mr. Tomcat 

然後Hibernate會遍歷每一行，解析ResultSet的每個單項並創建實體。是否有可能優化這種方法？由於嚴重的性能問題，我想通過子選擇選擇Cat#colors和Cat#nicknames集。在真實情況下，我獲取1500個具有複雜結構的實體，並不罕見，一個提取的實體在相應的ResultSet中生成25.000行，導致非常長的解析時間。

在我的情況下，延遲加載不是我想使用的選項，因爲它會給代碼帶來混亂。據我所知，延遲加載的Collection必須通過第一次調用初始化，這在我的真實應用程序中付出了相當大的可用性價格。

我將不勝感激3個單獨的選擇，其中一個來自cat表，一個來自cat_color表和一個來自cat_nickname表。

Answer 1

我發現Hibernate的解決方案，@Fetch(FetchMode.SELECT)做了這項工作，因爲它讓Hibernate通過單獨的select而不是join來選擇暱稱。

@Fetch(FetchMode.SELECT) 
@ElementCollection(fetch = FetchType.EAGER) 
@Column(name = "nickname") 
@CollectionTable(
     name = "cat_nickname", 
     joinColumns = @JoinColumn(name = "cat_id")) 
private Set<String> nicknames;