[英]How to optimize Carstesian Product when loading Hibernate Entity
為了演示我的問題,我創建了一個簡單的Spring Boot應用程序。 它具有一個具有ID的實體,兩個String
屬性和兩個Sets<String>
集。
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,可以持久保存並從數據庫中加載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());
}
}
我跟蹤了Hibernate,發現Cat
是通過此SQL從數據庫加載的。
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
然后,Hibernate從cat
表和兩個表的行中獲取此笛卡爾積,它們代表Cat#colors
和Cat#nicknames
集。
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
然后,Hibernate遍歷每一行,解析ResultSet的每一項並創建Entity。 是否可以通過某種方式優化此方法? 由於嚴重的性能問題,我想通過子選擇來選擇Cat#colors
和Cat#nicknames
集。 在實際情況下,我獲取了1500個結構復雜且並不罕見的實體,即一個獲取的實體在相應的ResultSet中生成25.000行,從而導致解析時間很長。
在我的情況下,延遲加載不是我想使用的選項,因為它會使代碼混亂。 據我所知,延遲加載的Collection必須通過首次調用進行初始化,這在我的實際應用程序中要付出很大的可用性。
我將不勝感激3個單獨的選擇,一個來自cat
表,一個來自cat_color
表,一個來自cat_nickname
表。
我找到了Hibernate的解決方案, @Fetch(FetchMode.SELECT)
完成了工作,因為它使Hibernate通過單獨的select(而不是join)來選擇昵稱。
@Fetch(FetchMode.SELECT)
@ElementCollection(fetch = FetchType.EAGER)
@Column(name = "nickname")
@CollectionTable(
name = "cat_nickname",
joinColumns = @JoinColumn(name = "cat_id"))
private Set<String> nicknames;
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