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What alternatives do I have to implement a union query using hibernate? I know hibernate does not support union queries at the moment, right now the only way I see to make a union is to use a view table.

The other option is to use plain jdbc, but this way I would loose all my example/criteria queries goodies, as well as the hibernate mapping validation that hibernate performs against the tables/columns.

Solution

Use VIEW. The same classes can be mapped to different tables/views using entity name, so you won't even have much of a duplication. Being there, done that, works OK.

Plain JDBC has another hidden problem: it's unaware of Hibernate session cache, so if something got cached till the end of the transaction and not flushed from Hibernate session, JDBC query won't find it. Could be very puzzling sometimes.

OTHER TIPS

You could use id in (select id from ...) or id in (select id from ...)

e.g. instead of non-working

from Person p where p.name="Joe"
union
from Person p join p.children c where c.name="Joe"

you could do

from Person p 
  where p.id in (select p1.id from Person p1 where p1.name="Joe") 
    or p.id in (select p2.id from Person p2 join p2.children c where c.name="Joe");

At least using MySQL, you will run into performance problems with it later, though. It's sometimes easier to do a poor man's join on two queries instead:

// use set for uniqueness
Set<Person> people = new HashSet<Person>((List<Person>) query1.list());
people.addAll((List<Person>) query2.list());
return new ArrayList<Person>(people);

It's often better to do two simple queries than one complex one.

EDIT:

to give an example, here is the EXPLAIN output of the resulting MySQL query from the subselect solution:

mysql> explain 
  select p.* from PERSON p 
    where p.id in (select p1.id from PERSON p1 where p1.name = "Joe") 
      or p.id in (select p2.id from PERSON p2 
        join CHILDREN c on p2.id = c.parent where c.name="Joe") \G
*************************** 1. row ***************************
           id: 1
  select_type: PRIMARY
        table: a
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 247554
        Extra: Using where
*************************** 2. row ***************************
           id: 3
  select_type: DEPENDENT SUBQUERY
        table: NULL
         type: NULL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: NULL
        Extra: Impossible WHERE noticed after reading const tables
*************************** 3. row ***************************
           id: 2
  select_type: DEPENDENT SUBQUERY
        table: a1
         type: unique_subquery
possible_keys: PRIMARY,name,sortname
          key: PRIMARY
      key_len: 4
          ref: func
         rows: 1
        Extra: Using where
3 rows in set (0.00 sec)

Most importantly, 1. row doesn't use any index and considers 200k+ rows. Bad! Execution of this query took 0.7s wheres both subqueries are in the milliseconds.

I have to agree with Vladimir. I too looked into using UNION in HQL and couldn't find a way around it. The odd thing was that I could find (in the Hibernate FAQ) that UNION is unsupported, bug reports pertaining to UNION marked 'fixed', newsgroups of people saying that the statements would be truncated at UNION, and other newsgroups of people reporting it works fine... After a day of mucking with it, I ended up porting my HQL back to plain SQL, but doing it in a View in the database would be a good option. In my case, parts of the query were dynamically generated, so I had to build the SQL in the code instead.

I have a solution for one critical scenario (for which I struggled a lot )with union in HQL .

e.g. Instead of not working :-

select i , j from A a  , (select i , j from B union select i , j from C) d where a.i = d.i 

OR

select i , j from A a  JOIN (select i , j from B union select i , j from C) d on a.i = d.i 

YOU could do in Hibernate HQL ->

Query q1 =session.createQuery(select i , j from A a JOIN B b on a.i = b.i)
List l1 = q1.list();

Query q2 = session.createQuery(select i , j from A a JOIN C b on a.i = b.i)
List l2 = q2.list();

then u can add both list ->

l1.addAll(l2);

A view is a better approach but since hql typically returns a List or Set... you can do list_1.addAll(list_2). Totally sucks compared to a union but should work.

Perhaps I had a more straight-forward problem to solve. My 'for instance' was in JPA with Hibernate as the JPA provider.

I split the three selects (two in a second case) into multiple select and combined the collections returned myself, effectively replacing a 'union all'.

I too have been through this pain - if the query is dynamically generated (e.g. Hibernate Criteria) then I couldn't find a practical way to do it.

The good news for me was that I was only investigating union to solve a performance problem when using an 'or' in an Oracle database.

The solution Patrick posted (combining the results programmatically using a set) while ugly (especially since I wanted to do results paging as well) was adequate for me.



As Patrick said, appending the LISTs from each SELECT would be a good idea but remember that it acts like UNION ALL. To avoid this side effect, just control if the object is already added in final collection or not. If no, then add it.
Something else that you should care about is that if you have any JOIN in each SELECT, the result would be a list of object array(List<Objetc[]>) so you have to iterate over it to only keep the object that you need.

Hope it works.

Here is a special case, but might inspire you to create your own work around. The goal here is to count the total number of records from two different tables where records meet a particular criteria. I believe this technique will work for any case where you need to aggregate data from across multiple tables/sources.

I have some special intermediate classes setup, so the code which calls the named query is short and sweet, but you can use whatever method you normally use in conjunction with named queries to execute your query.

QueryParms parms=new QueryParms();
parms.put("PROCDATE",PROCDATE);

Long pixelAll = ((SourceCount)Fetch.row("PIXEL_ALL",parms,logger)).getCOUNT();

As you can see here, the named query begins to look an aweful lot like a union statement:

@Entity
@NamedQueries({
        @NamedQuery(
            name  ="PIXEL_ALL",
            query = "" +
                    "  SELECT new SourceCount(" +
                    "     (select count(a) from PIXEL_LOG_CURR1 a " +
                    "       where to_char(a.TIMESTAMP, 'YYYYMMDD') = :PROCDATE " +
                    "     )," +
                    "     (select count(b) from PIXEL_LOG_CURR2 b" +
                    "       where to_char(b.TIMESTAMP, 'YYYYMMDD') = :PROCDATE " +
                    "     )" +
                    ") from Dual1" +
                    ""
    )
})

public class SourceCount {
    @Id
    private Long   COUNT;

    public SourceCount(Long COUNT1, Long COUNT2) {
        this.COUNT = COUNT1+COUNT2;
    }

    public Long getCOUNT() {
        return COUNT;
    }

    public void setCOUNT(Long COUNT) {
        this.COUNT = COUNT;
    }
}

Part of the magic here is to create a dummy table and insert one record into it. In my case, I named it dual1 because my database is Oracle, but I don't think it matters what you call the dummy table.

@Entity
@Table(name="DUAL1")
public class Dual1 {
    @Id
    Long ID;
}

Don't forget to insert your dummy record:

SQL> insert into dual1 values (1);

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