Difference between join fetch Hsql and standard SQL request.

Introduction

I stumbled upon a problem with a HSQL request recently(Hibernate SQL language). The application using a Hsql request was not retrieving the information we wanted and we did not know why.

The SQL request retrieving correctly the information

The tables from the database(simplified) :


ALARM                                                         
        ID          NAME    
        122        NAME_X


MY_TABLE
        MY_ID     MY_NAME       VarA 
        231       NAME_Y        value_expected
        Null      NAME_X        value_expected

This SQL request executed from SQL developer would retrieve the expected outcome from the tables.

SELECT a FROM MY_TABLE t, ALARM a WHERE
t.VarA = 'value_expected' and
a.NAME=t.MY_NAME and (a.ID=t.MY_ID or a.ID is null).

Outcome :

Null           NAME_Y

The different behavior with looking similar HSQL request

The HSQL request( used with entities) in the java code was supposed to give the same results as the previous sql request. Here is the code of the hsql request :

StringBuilder clause = new StringBuilder(
"from MyAlarm a join fetch a.mytable t"
+" where t.VarA = 'value_expected' and"
+"(a.ID=t.MY_ID or a.ID is null)");

The problem when using this hsql request is that some of the request is done through entity objects (from MyAlarm a join fetch a.mytabl). The join is done in the entity class of the Entity MyAlarm :

@Entity
@Table(name = "MY_ALARM")
@Cache(usage = CacheConcurrencyStrategy.NONE)
public class MyAlarm implements Serializable {
    @Column(name = "NAME")
    private String Name;
    @Column(name = "ID")
    private String Id;

 @ManyToOne(targetEntity=MyTable.class, optional = true, fetch=FetchType.LAZY)
    <b>@JoinColumns({
        @JoinColumn(name="NAME",  
referencedColumnName="MY_NAME"     
,nullable = true, insertable = false
, updatable = false),
        @JoinColumn(name="ID",    
referencedColumnName="MY_ID"     
,nullable = true
, insertable = false
, updatable = false)</b>
    })

This following join from the entity will never retrieve the row of the table MY_TABLE where ID is null because there is no ID null in the table ALARM. Therefore this hsql request is not not equivalent to the SQL request above.

Explanation about join fetch

The documentation about join fetch : “14.3. Associations and joins”
https://docs.jboss.org/hibernate/orm/3.3/reference/en/html/queryhql.html

A good explanation of the difference between join and join fetch here :
http://stackoverflow.com/questions/17431312/difference-between-join-and-join-fetch-in-hibernate

How did I find the solution ?

As i said previously unit testing is the key to find the resolution of coding problems. It enables you to reproduce problems faster and it adds a regression test to your product. More information about unit testing https://julienprog.wordpress.com/2015/03/15/the-power-of-unit-testing/

I reproduced the problem inside a unit test.It helped me a lot to understand the problem and how to solve it. To realise a unit test of hibernate request i use hsqldb,spring framework ,maven,etc..

The solution

I join the two tables “MyAlarm” and “MyTable” with the following conditions. The important part was to add (a.ID=t.MY_ID or a.ID is null). It is a condition necessary to respect the needs of the customer.

<b>select a from MyAlarm a, MyTable t</b> where 
t.VarA = 'value_expected' and
a.NAME=t.MY_NAME (a.ID=t.MY_ID or a.ID is null)
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