Why does Hibernate generate a CROSS JOIN for an implicit join of a @ManyToOne association?

Baur & King said in their book:

Implicit joins are always directed along many-to-one or one-to-one association, never through a collection-valued association.

[P 646, Ch 14]

But when I am doing that in the code it is generating a CROSS JOIN instead of an INNER JOIN.

Mapping is from Member2 (many-to-one) -> CLub.

But Club2 has no information about members and Member2 is having a Foreign Key of Club2.

My query is

// Implicit: Find all UK club member who is female
Transaction t1 = HibernateUtil.begin();
Query query =
    HibernateUtil.getSession().createQuery("From Member2 m where m.club2.country = 'UK' ");
List<Member2> memList = query.list();
for (Member2 m : memList)
  System.out.println(m);
HibernateUtil.end(t1);

And, Hibernate is generating the following SQL query:

Hibernate: 
    select
        member2x0_.member_id as member_i1_1_,
        member2x0_.club_id as club_id5_1_,
        member2x0_.member_age as member_a2_1_,
        member2x0_.member_name as member_n3_1_,
        member2x0_.member_sex as member_s4_1_ 
    from
        TBL_MEMBER2 member2x0_ cross 
    join
        TBL_CLUB2 club2x1_ 
    where
        member2x0_.club_id=club2x1_.club_id 
        and club2x1_.country='UK'
Hibernate: 
    select
        club2x0_.club_id as club_id1_0_0_,
        club2x0_.club_name as club_nam2_0_0_,
        club2x0_.country as country3_0_0_ 
    from
        TBL_CLUB2 club2x0_ 
    where
        club2x0_.club_id=?
aaa 25 m
bbb 28 f

Club2.java

package com.lilu.de.onetomany.uni.other;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.SequenceGenerator;
import javax.persistence.Table;

@Entity
@Table(name = "TBL_CLUB2")
public class Club2 {

  @GeneratedValue(generator = "pkey_Club2", strategy = GenerationType.SEQUENCE)
  @SequenceGenerator(name = "pkey_Club2", initialValue = 1000, allocationSize = 10,
      sequenceName = "seq_pkey_Club2")
  @Id
  private int club_id;

  private String club_name;

  private String country;

  // private Set Member2 = new HashSet();

  public Club2() {
    super();
  }

  public Club2(String cname, String ccountry) {
    this.club_name = cname;
    this.country = ccountry;
  }

  @Override
  public String toString() {
    String temp = club_name + " " + country + " ";
    // Iterator<Member2> iter = Member2.iterator();
    // String mems = null;
    // while (iter.hasNext()) {
    // mems += iter.next();
    // }
    // temp += "n" + mems;
    return temp;
  }

  public int getClub_id() {
    return club_id;
  }

  public void setClub_id(int club_id) {
    this.club_id = club_id;
  }

  public String getClub_name() {
    return club_name;
  }

  public void setClub_name(String club_name) {
    this.club_name = club_name;
  }

  public String getCountry() {
    return country;
  }

  public void setCountry(String country) {
    this.country = country;
  }

  /*
   * public Set<Member2> getMember2() { return Member2; }
   * 
   * public void setMember2(Set<Member2> Member2) { this.Member2 = Member2; }
   */
}

Member2.java

package com.lilu.de.onetomany.uni.other;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.SequenceGenerator;
import javax.persistence.Table;

@Entity
@Table(name = "TBL_MEMBER2")
public class Member2 {

  @GeneratedValue(generator = "pkey_member2", strategy = GenerationType.SEQUENCE)
  @SequenceGenerator(name = "pkey_member2", sequenceName = "seq_pkey_member2")
  @Id
  private int member_id;

  private String member_name;

  private int member_age;

  private char member_sex;

  @ManyToOne(cascade = CascadeType.ALL)
  @JoinColumn(name = "club_id")
  private Club2 club2;

  public Member2() {
    super();
  }

  public Member2(String mname, int age, char sex) {
    this.member_name = mname;
    this.member_age = age;
    this.member_sex = sex;
  }

  public Club2 getClub2() {
    return club2;
  }

  public void setClub2(Club2 club2) {
    this.club2 = club2;
  }

  @Override
  public String toString() {
    return member_name + " " + member_age + " " + member_sex;
  }

  public String getMember_name() {
    return member_name;
  }

  public void setMember_name(String member_name) {
    this.member_name = member_name;
  }

  public int getMember_age() {
    return member_age;
  }

  public void setMember_age(int member_age) {
    this.member_age = member_age;
  }

  public char getMember_sex() {
    return member_sex;
  }

  public void setMember_sex(char member_sex) {
    this.member_sex = member_sex;
  }

  public int getMember_id() {
    return member_id;
  }

  public void setMember_id(int member_id) {
    this.member_id = member_id;
  }

}

hibernate.cfg.xml

<?xml version='1.0' encoding='utf-8'?>
<!--
  ~ Hibernate, Relational Persistence for Idiomatic Java
  ~
  ~ Copyright (c) 2010, Red Hat Inc. or third-party contributors as
  ~ indicated by the @author tags or express copyright attribution
  ~ statements applied by the authors.  All third-party contributions are
  ~ distributed under license by Red Hat Inc.
  ~
  ~ This copyrighted material is made available to anyone wishing to use, modify,
  ~ copy, or redistribute it subject to the terms and conditions of the GNU
  ~ Lesser General Public License, as published by the Free Software Foundation.
  ~
  ~ This program is distributed in the hope that it will be useful,
  ~ but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  ~ or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License
  ~ for more details.
  ~
  ~ You should have received a copy of the GNU Lesser General Public License
  ~ along with this distribution; if not, write to:
  ~ Free Software Foundation, Inc.
  ~ 51 Franklin Street, Fifth Floor
  ~ Boston, MA  02110-1301  USA
  -->
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

    <session-factory>

        <!-- Database connection settings -->
        <property name="connection.driver_class">org.postgresql.Driver</property>
        <property name="connection.url">jdbc:postgresql://localhost:5432/hibernatedb1</property>
        <property name="connection.username">postgres</property>
        <property name="connection.password">ani155</property>

        <!-- JDBC connection pool (use the built-in) -->
        <property name="connection.pool_size">1</property>

        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.PostgreSQLDialect</property>

        <!-- Disable the second-level cache  -->
        <property name="cache.provider_class">org.hibernate.cache.internal.NoCacheProvider</property>

        <!-- Echo all executed SQL to stdout -->
        <property name="show_sql">true</property>
        <property name="format_sql">true</property> 

        <!-- Drop and re-create the database schema on startup -->
        <property name="hbm2ddl.auto">update</property>

        <!-- Names the annotated entity class -->
        <mapping class="com.lilu.de.onetomany.uni.other.Club2"/>
        <mapping class="com.lilu.de.onetomany.uni.other.Member2"/>
       <!--   <mapping class="com.apal.mapping.onetoone.User"/> -->
    </session-factory>

</hibernate-configuration>

Test.java

package com.lilu.de.onetomany.uni.other;

import java.util.List;

import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;

public class Test {
  public static void main(String[] args) {

    Test test = new Test();
    // test.setup();
    test.SelectQuery1Implicit();
    // test.SelectQuery2ExplicitFromClause();
    // test.SelectQuery3JoinFetch();
    // test.SelectQuery4ThetaJoin();
  }

  private void SelectQuery1Implicit() {
    // Implicit: Find all UK club member who is female
    Transaction t1 = HibernateUtil.begin();
    Query query =
        HibernateUtil.getSession().createQuery("From Member2 m where m.club2.country = 'UK' ");
    List<Member2> memList = query.list();
    for (Member2 m : memList)
      System.out.println(m);
    HibernateUtil.end(t1);
  }

  private void setup() {
    Transaction t1 = HibernateUtil.begin();
    Club2 c1 = new Club2("MulaRougne", "UK");
    Member2 m1 = new Member2("aaa", 25, 'm');
    Member2 m2 = new Member2("bbb", 28, 'f');
    m1.setClub2(c1);
    m2.setClub2(c1);
    HibernateUtil.getSession().save(c1);
    HibernateUtil.getSession().save(m1);
    HibernateUtil.getSession().save(m2);
    Club2 c2 = new Club2("Queen's Club", "UK");
    Club2 c3 = new Club2("Disney", "USA");
    Member2 m3 = new Member2("ccc", 32, 'm');
    Member2 m4 = new Member2("ddd", 23, 'm');
    m3.setClub2(c3);
    m4.setClub2(c3);
    HibernateUtil.getSession().save(m3);
    HibernateUtil.getSession().save(m4);
    HibernateUtil.getSession().save(c2);
    HibernateUtil.getSession().save(c3);


    /*
     * Club2 c1 = new Club2("MulaRougne", "UK"); Club2 c2 = new Club2("Queen's Club", "UK"); Club2
     * c3 = new Club2("Disney", "USA");
     * 
     * Member2 m1 = new Member2("aaa", 25, 'm'); Member2 m2 = new Member2("bbb", 28, 'f'); Member2
     * m3 = new Member2("ccc", 32, 'm'); Member2 m4 = new Member2("ddd", 23, 'm'); Member2 m5 = new
     * Member2("ee", 30, 'f');
     * 
     * c1.getMember2().add(m1); c1.getMember2().add(m2); c1.getMember2().add(m3);
     * 
     * c2.getMember2().add(m4); c2.getMember2().add(m5);
     * 
     * HibernateUtil.getSession().save(c2); HibernateUtil.getSession().save(m4);
     * HibernateUtil.getSession().save(m5);
     * 
     * HibernateUtil.getSession().save(c1); HibernateUtil.getSession().save(m1);
     * HibernateUtil.getSession().save(m2); HibernateUtil.getSession().save(m3);
     */
    HibernateUtil.end(t1);
  }


  private static class HibernateUtil {
    private static SessionFactory factory;
    private static Session session;
    static {
      factory = new Configuration().configure().buildSessionFactory();
    }

    public static Session getSession() {
      return session;
    }

    public static Transaction begin() {
      session = factory.openSession();
      return session.beginTransaction();
    }

    public static void end(Transaction tran) {
      tran.commit();
    }
  }
}

Answer

Most database engines will optimize the CROSS JOIN with a WHERE clause to a JOIN anyway, but I prefer to always use an explicit JOIN instead.

The CROSS JOIN is generated by the JOIN:

where m.club2.country = 'UK'

To avoid the second Club query you could write the query as follows:

Query query = session.createQuery("""
        select m 
        from Member2 m
        join fetch m.club2 c
        where
           c.country = :country
        """, Member2.class)
.setParameter("country", "UK");

This query will remove the CROSS JOIN and the secondary select while using bind parameters instead of hard-coded ones.