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Make Hibernate discriminator value use bind variable instead of literal

When using the #DiscriminatorColumn on a class and #DiscriminatorValue on subclasses, the SQL that Hibernate generates uses the discriminator value as a literal for the clause involving the discriminator column.
Example:
#Entity
#Inheritance(strategy = InheritanceType.SINGLE_TABLE)
#DiscriminatorColumn(name = "PART_TYPE")
#Table(name = "NAME")
public class NamePartEntity implements Comparable<NamePartEntity> {
// Stuff
}
#Entity
#DiscriminatorValue(value = "GIV")
public class GivenNameEntity extends NamePartEntity {
// stuff
}
If I create a simple criteria query with no criteria except the class, I would get SQL generated like such:
select this_.person_id as y0_ from name this_ where this_.part_type='GIV'
This isn't so bad until you have a handful of discriminator values and the table might be selected from multiple times, such that a query like the below:
SELECT this_.person_id AS y0_
FROM name this_
WHERE this_.part_type='FAM'
AND this_.value_u =:8
AND this_.tenant_id =:9
AND this_.person_id IN
(SELECT this_.person_id AS y0_
FROM name this_
WHERE this_.part_type='GIV'
AND this_.value_u =:10
AND this_.tenant_id =:11
AND this_.person_id IN
(SELECT this_.person_id AS y0_
FROM name this_
WHERE this_.part_type='GIV'
AND this_.value_u =:12
AND this_.tenant_id =:13
AND this_.person_id IN
(SELECT this_.person_id AS y0_
FROM name this_
WHERE this_.part_type='PFX'
AND this_.value_u =:14
AND this_.tenant_id =:15
)
)
)
could have a ton of different SQL ids and execution plans based on the literals ('FAM', 'GIV', 'PFX' in this case but they could be different and in different orders). However, if bind variables were used in place of those discriminator value literals, it would be the same sql id and have the same execution plan.
So, is it possible to have Hibernate use the discriminator column/value annotations in such a way that bind variables are used instead of literals? I know it would be possible to rewrite my entities in such a way to avoid this but I wanted to see if I could get the bind variable functionality with the existing annotations in some way.
Alternatively, is there a way I can still use my extended classes without using discriminator values? If I try that and have the #Entity annotation on each extended class, it complains about missing the discriminator type even when there are no discriminator annotations.

No, it is not possible to get it out of the box.
The closest workaround that comes to my mind is to select from the base class and explicitly filter by subclass:
entityManager.createQuery("select gne from NamePartEntity gne where type(gne) = :subclass")
.setParameter("subclass", GivenNameEntity.class)
.getResultList();

Alternatively, is there a way I can still use my extended classes
without using discriminator values? If I try that and have the #Entity
annotation on each extended class, it complains about missing the
discriminator type even when there are no discriminator annotations
That is because of the InheritanceType.SINGLE_TABLE strategy you've used. You could always try to use other strategies, like InheritanceType.JOINED, depending upon what you query for most often.
On another note, Doesn't having this many types of Names per-se an indicator that you must have types as a relation and not a Sub-type?

How can I work-around this Hibernate Limitation?

I have 3 entities in a Hierarchy like this:
MyInterface
|
-----------------
| |
Entity1 Entity2
The MyInterface is NOT mapped in Hibernate (because I am using implicit polymorphism strategy to map this inheritance)
And, in fact, if I launch a query like this one:
"FROM MyInterface"
It works just fine (because it retrieves all the instances of Entity1 and all the instances of Entity2, puts them together, and returns a List<MyInterface>).
If we look at the SQL generated by Hibernate, it is launching 2 independent SQL queries to first retrieve the Entity1 instances an another one to retrieve Entity2 instances, but I am fine with this.
The BIG problem comes when you try to do something like this:
"FROM MyInterface ORDER BY someField"
Because it is applying the ORDER BY to the first SQL query and then the same ORDER BY to the second SQL query, instead of apply them to the WHOLE query (I know this because I can see the native SQL queries launched by Hibernate).
This is clearly a missbehaviour of Hibernate.
How can I work around this to force Hibernate to apply the ORDER BY to the whole query? (I cannot do it in memory because later I will have to add pagination also).

I'd say the problem is that Hibernate has to create those 2 SQL queries because you have to read from 2 tables.
I'm not sure if reading from 2 tables and ordering by 2 columns (one from each table) in one query is possible in plain SQL (means no vendor specific extensions), and if not, Hibernate would have to do the ordering in memory anyways.
What you could do when applying paging is: read the ids and the values you want to sort by only (not the entire entity), then sort in memory and read the entire entity for all ids contained in the page. For paging to be consistent you might have to store the results of that initial query (id + order criteria) anyways.

The way you are thinking cannot be mapped to SQL as is. Suppose you have Entity1 with fields field1A, field1B ... and Entity2 with fields field2A, field2B, ... Now you want the following query to be executed:
SELECT Entity1.* FROM Entity1
UNION
SELECT Entity2.* FROM Entity2
ORDER BY CommonField
which is not possible in SQL world, as entities have different number of fields and different field types.
So you need to think about extracting common fields into separate table CommonEntity, converting your interface into standalone entity with with one-to-one mapping to Entity1 & Entity2 (see Table per subclass). Then SQL will look like:
SELECT * from CommonEntity LEFT OUTER JOIN Entity1 ON Entity1.refId = CommonEntity.id LEFT OUTER JOIN Entity2 ON Entity2.refId = CommonEntity.id
ORDER BY CommonField
Or you can create a view over your tables and introduce an artificial discriminator (discriminator is something which will "distinguish" IDs from different tables, which caused a problem in your solution) and then map an entity to this view (so we get Table per class hierarchy):
CREATE VIEW EntityAandEntityB AS
SELECT 'A' as discriminator, Entity1.ID, CommonField1, ... CommonFieldZ, Entity1.field1A, ... Entity1.field1N, NULL, NULL, ... NULL(M)
FROM Entity1
UNION
SELECT 'B' as discriminator, Entity2.ID, CommonField1, ... CommonFieldZ, NULL, NULL, ... NULL(N), Entity2.field2A, ... Entity2.field2M
FROM Entity2
ORDER BY CommonField1, ...
Other alternatives (e.g. mentioned by #UdoFholl which is also kind of "outer join" for EntityAandEntityB) will result 2 SQLs and thus there is no way to order the "whole" query, and scrolling is not possible.

Hibernate will do this for you if you use the following.
#Entity
#Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
public abstract class AbstractEntity implements MyInterface {
private int someField;
}
Then have your subclasses that do this
#Entity
#Table(name="entity_1")
public class EntityOne extends AbstractEntity {
private int someOtherField;
}
#Entity
#Table(name="entity_2")
public class EntityTwo extends AbstractEntity {
private int anotherSomeOtherField;
}
You should then be able to write a query like this to get a single union SQL query with the DB doing the ordering.
FROM AbstractEntity ORDER BY someField

How to write a HQL query for Many To Many Associations?

I have 3 tables, Role[roleId, roleName], Token[tokenID, tokenName] & ROLETOKENASSOCIATION[roleId, tokenID]. The 3rd one was created automatically by hibernate. Now if i simply write a Query to get all the objects from Role class means, it gives the all role objects along with the associated tokenID & tokenName.
I just wanted the association as unidirectional. i.e: Roles--->Tokens
So the annotation in the Role class looks like,
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
private int roleId;
private String roleName;
#ManyToMany
#JoinTable(name="ROLE_TOKEN_ASSOCIATION",
joinColumns={#JoinColumn(name="roleId")},
inverseJoinColumns={#JoinColumn(name="tokenID")})
private List<Token> tkns;
//Getters & Setters
Now i want the tokenNames for the specific roleId.
First i made a query like this SELECT tkns.tokenName FROM Role WHERE Role.roleId=:roleId
But, i ended up with some dereference error.
Then i changed the query to SELECT tkns FROM Role r WHERE r.roleId=:roleId
Now i have got what i wanted. But it comes with roleId too.
How shall i get tokenName itself?
Actually my problem is solved, but i would like to know how to do it.
It ll be helpful to me, if anyone explains the Query Construction.
Any suggestions!!

Have you tried
SELECT t.tokenName FROM Role r JOIN r.tkns t WHERE r.roleId = :roleId
EDIT: This query almost directly maps to the corresponding SQL query where Role r JOIN r.tkns t is a shorthand syntax for the SQL join via the link table Role r JOIN ROLETOKENASSOCIATION rt ON r.roleId = rt.roleId JOIN Token ON rt.tokenId = t.tokenId. Affe's answer is another syntax for the same query.
See also:
Chapter 14. HQL: The Hibernate Query Language

You want a scalar list of just the name field? You should be able to get that like this
select t.name from Roles r, IN(r.tkns) t where r.roleId = :id

Hibernate Subquery Question

This should be a simple one I hope.
I have an invoice and that invoice has a list of payments.
Using the Criteria API I am trying to return a list of invoices and their payment total. So, in SQL I want something like this:
SELECT i.*, (SELECT SUM(PMT_AMOUNT) FROM INVOICE_PAYMENTS p WHERE p.INVOICE = i.INVOICE) FROM INVOICES i
I can't for the life of me figure out how to achieve this with the Criteria API. Doing something like:
Criteria crit = session.createCriteria(Invoice.class)
criteria.setProjection(Projections.projectionList()
.add(Projections.sum("payements.paymentAmount").as("paymentTotal"))
Simply returns 1 row with the projected payment total for all invoices, which is actually what you'd expect, but this is as close as I can get.
Any help is greatly appreciated.

There is a way with Criteria to return a list of Invoices along with the total payments for that invoice.
In theory, the answer is that you can use a grouping property on a projection query to group the result into total payment by invoice. The second part is that you could use a transient "totalPayment" value on the Invoice and select the projection into the Invoice structure using a transformer. This would be easier than dealing with an ArrayList of different properties but would depend on what you needed to use the results for.
To demonstrate this, here is the important part of a small Invoice class:
public class Invoice{
private String name;
#Transient private int totalPayments;
#OneToMany Set<Payment> payments = new HashSet<Payment>();
// getters and setters
...
}
Then this is the criteria that you could use
Criteria criteria = session.createCriteria(Invoice.class)
.createAlias("payments", "pay")
.setProjection(Projections.projectionList()
.add(Projections.groupProperty("id"))
.add(Projections.property("id"), "id")
.add(Projections.property("name"), "name")
.add(Projections.sum("pay.total").as("totalPayments")))
.setResultTransformer(Transformers.aliasToBean(Invoice.class));
List<Invoice> projected = criteria.list();
And this is the sql that is generated
Hibernate:
select this_.id as y0_,
this_.id as y1_,
this_.name as y2_,
sum(pay1_.total) as y3_
from invoice this_
inner join invoice_payment payments3_ on this_.id=payments3_.invoice_id
inner join payment pay1_ on payments3_.payments_id=pay1_.id
group by this_.id

I'm pretty sure you can't return entities in a Projection.
There are two possibles:
Run two criteria queries, one for the actual invoices and one for there totals
Use HQL to perform the query
I haven't tested this but it should go something like:
select i, (select sum(p.amount) from InvoicePayments p where p.invoice = i.invoice) from Invoice i
Will have to wait until tomorrow, I have a very similar data structure at work I should be able to test this then.

You can also use #Formula for the totalPayments field. Disadvantage is, that the "sum" is computed every time you load the entity. So, you may use LAZY #Formula - do build time enhancement or Pawel Kepka's trick: http://justonjava.blogspot.com/2010/09/lazy-one-to-one-and-one-to-many.html Disadvantage is, that is you have more LAZY #Fromula and you hit just one of them, all of them are loaded. Another solution may be to use #MappedSuperclass and more subclasses. Each subclass may have different #Formula fields. And one more solution beside DB view: Hibernate #Subselect.

JPA eager fetch does not join

What exactly does JPA's fetch strategy control? I can't detect any difference between eager and lazy. In both cases JPA/Hibernate does not automatically join many-to-one relationships.
Example: Person has a single address. An address can belong to many people. The JPA annotated entity classes look like:
#Entity
public class Person {
#Id
public Integer id;
public String name;
#ManyToOne(fetch=FetchType.LAZY or EAGER)
public Address address;
}
#Entity
public class Address {
#Id
public Integer id;
public String name;
}
If I use the JPA query:
select p from Person p where ...
JPA/Hibernate generates one SQL query to select from Person table, and then a distinct address query for each person:
select ... from Person where ...
select ... from Address where id=1
select ... from Address where id=2
select ... from Address where id=3
This is very bad for large result sets. If there are 1000 people it generates 1001 queries (1 from Person and 1000 distinct from Address). I know this because I'm looking at MySQL's query log. It was my understanding that setting address's fetch type to eager will cause JPA/Hibernate to automatically query with a join. However, regardless of the fetch type, it still generates distinct queries for relationships.
Only when I explicitly tell it to join does it actually join:
select p, a from Person p left join p.address a where ...
Am I missing something here? I now have to hand code every query so that it left joins the many-to-one relationships. I'm using Hibernate's JPA implementation with MySQL.
Edit: It appears (see Hibernate FAQ here and here) that FetchType does not impact JPA queries. So in my case I have explicitly tell it to join.

JPA doesn't provide any specification on mapping annotations to select fetch strategy. In general, related entities can be fetched in any one of the ways given below
SELECT => one query for root entities + one query for related mapped entity/collection of each root entity = (n+1) queries
SUBSELECT => one query for root entities + second query for related mapped entity/collection of all root entities retrieved in first query = 2 queries
JOIN => one query to fetch both root entities and all of their mapped entity/collection = 1 query
So SELECT and JOIN are two extremes and SUBSELECT falls in between. One can choose suitable strategy based on her/his domain model.
By default SELECT is used by both JPA/EclipseLink and Hibernate. This can be overridden by using:
#Fetch(FetchMode.JOIN)
#Fetch(FetchMode.SUBSELECT)
in Hibernate. It also allows to set SELECT mode explicitly using #Fetch(FetchMode.SELECT) which can be tuned by using batch size e.g. #BatchSize(size=10).
Corresponding annotations in EclipseLink are:
#JoinFetch
#BatchFetch

"mxc" is right. fetchType just specifies when the relation should be resolved.
To optimize eager loading by using an outer join you have to add
#Fetch(FetchMode.JOIN)
to your field. This is a hibernate specific annotation.

The fetchType attribute controls whether the annotated field is fetched immediately when the primary entity is fetched. It does not necessarily dictate how the fetch statement is constructed, the actual sql implementation depends on the provider you are using toplink/hibernate etc.
If you set fetchType=EAGER This means that the annotated field is populated with its values at the same time as the other fields in the entity. So if you open an entitymanager retrieve your person objects and then close the entitymanager, subsequently doing a person.address will not result in a lazy load exception being thrown.
If you set fetchType=LAZY the field is only populated when it is accessed. If you have closed the entitymanager by then a lazy load exception will be thrown if you do a person.address. To load the field you need to put the entity back into an entitymangers context with em.merge(), then do the field access and then close the entitymanager.
You might want lazy loading when constructing a customer class with a collection for customer orders. If you retrieved every order for a customer when you wanted to get a customer list this may be a expensive database operation when you only looking for customer name and contact details. Best to leave the db access till later.
For the second part of the question - how to get hibernate to generate optimised SQL?
Hibernate should allow you to provide hints as to how to construct the most efficient query but I suspect there is something wrong with your table construction. Is the relationship established in the tables? Hibernate may have decided that a simple query will be quicker than a join especially if indexes etc are missing.

Try with:
select p from Person p left join FETCH p.address a where...
It works for me in a similar with JPA2/EclipseLink, but it seems this feature is present in JPA1 too:

If you use EclipseLink instead of Hibernate you can optimize your queries by "query hints". See this article from the Eclipse Wiki: EclipseLink/Examples/JPA/QueryOptimization.
There is a chapter about "Joined Reading".

to join you can do multiple things (using eclipselink)
in jpql you can do left join fetch
in named query you can specify query hint
in TypedQuery you can say something like
query.setHint("eclipselink.join-fetch", "e.projects.milestones");
there is also batch fetch hint
query.setHint("eclipselink.batch", "e.address");
see
http://java-persistence-performance.blogspot.com/2010/08/batch-fetching-optimizing-object-graph.html

I had exactly this problem with the exception that the Person class had a embedded key class.
My own solution was to join them in the query AND remove
#Fetch(FetchMode.JOIN)
My embedded id class:
#Embeddable
public class MessageRecipientId implements Serializable {
#ManyToOne(targetEntity = Message.class, fetch = FetchType.LAZY)
#JoinColumn(name="messageId")
private Message message;
private String governmentId;
public MessageRecipientId() {
}
public Message getMessage() {
return message;
}
public void setMessage(Message message) {
this.message = message;
}
public String getGovernmentId() {
return governmentId;
}
public void setGovernmentId(String governmentId) {
this.governmentId = governmentId;
}
public MessageRecipientId(Message message, GovernmentId governmentId) {
this.message = message;
this.governmentId = governmentId.getValue();
}
}

Two things occur to me.
First, are you sure you mean ManyToOne for address? That means multiple people will have the same address. If it's edited for one of them, it'll be edited for all of them. Is that your intent? 99% of the time addresses are "private" (in the sense that they belong to only one person).
Secondly, do you have any other eager relationships on the Person entity? If I recall correctly, Hibernate can only handle one eager relationship on an entity but that is possibly outdated information.
I say that because your understanding of how this should work is essentially correct from where I'm sitting.