当Hibernate使用JOIN和Single Queries时

Question

我有两个表，还有ManyToOne和fetch Eager一起加入。 但仍然Hibernate正在触发单个查询而不是JOIN。

hibernate如何决定何时使用JOIN以及何时使用单个查询？

我的意思是即使我们使用LAZY，有时候hibernate会激活JOIN查询而不是单个查询。 使用Eager，它应该激活JOIN，但有时会触发单个查询。

这是我无法理解的？

表1

@Entity
@Table(name="localityCityMapping")
public class LocalityCityMapping implements Serializable {
    private static final long serialVersionUID = 1L;

@Id
private int localityId;

private int cityId;

private String localityName;

private String status;

private Timestamp submitDate;

private int zoneId;

@OneToOne(fetch=FetchType.LAZY)
@JoinColumn(name="cityId", referencedColumnName="city_id", insertable=false, updatable=false)
private CountryCityTable countryCityData;

...

}

表2

@Entity
@Table(name="countryCityTable")
public class CountryCityTable implements Serializable {
    private static final long serialVersionUID = 1L;

@Id
@Column(name="city_id")
private int cityId;

private int addedBy;

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

private int countryId;

private Timestamp creationDate;

private int enabled;

private String notes;


@Column(name="state_id")
private int stateId;

private byte tier;

}

Hibernate: select localityci0_.localityId as locality1_2_, localityci0_.cityId as cityId2_2_, localityci0_.localityName as locality3_2_, localityci0_.status as status4_2_, localityci0_.submitDate as submitDa5_2_, localityci0_.zoneId as zoneId6_2_ from localityCityMapping localityci0_ where localityci0_.localityId in (? , ?)

Hibernate: select countrycit0_.city_id as city_id1_0_0_, countrycit0_.addedBy as addedBy2_0_0_, countrycit0_.city_name as city_nam3_0_0_, countrycit0_.countryId as countryI4_0_0_, countrycit0_.creationDate as creation5_0_0_, countrycit0_.enabled as enabled6_0_0_, countrycit0_.notes as notes7_0_0_, countrycit0_.state_id as state_id8_0_0_, countrycit0_.tier as tier9_0_0_ from countryCityTable countrycit0_ where countrycit0_.city_id=?

相同的查询即使加入是EAGER / LAZY。