Hi!

I have read the book "SQL Tuning" by Dan Tow and want to optimise the important HQL/JPA-QL queries of my application, but I not sure how to enforce the optimal execution plan :-(.

Consider the following JPA-QL-Query:

| select o
| from Operation o
| where o.order.flight.etd = :flight
|

The resulting SQL-Statement:

| select
|  ....
| from
|  Operation operation0_,
|  OrderTab order1_,
|  Flight flight2_
| where
|  order1_.flight_id=flight2_.id
|  and operation0_.order_id=order1_.id
|  and flight2_.etd=?
|  

Let's say I want to get the following join order: Flight, OrderTab, Operation. How can I get it? Exists there a way to enforce this join order with Jpa-QL or HQL?
If it were possible to enforce the join order with JPA-QL I would only have to change the JPA-QL query, the other application code would remain unchanged:

| Query query = em.createQuery(<JPA-QL Query which enforces the join order>);
| Operation op = (Operation)em.getSingleResult();
| ....
|

But if the only way to enforce the join order is to execute a sql query instead the HQL-Query, I think would have to write the following code:

| Query query = em.createNativeQuery(<SQL Query which enforces the join order>);
| Object[] ar = (Object[])em.getSingleResult(); /*the result of the query won't be an object with the type Operation but an array of objects (the attribute values)*/
| Operation op = (Operation)new Operation().setAttr1(ar[0]).setAttr2(ar[1]) ...;
|

The code above is very clumsy and not very convenient. If sql is the only way to enforce the wanted execution plan, can I let hibernate transform the result of the query into an Operation-object?
The code changes in this case would be minimal:

| Query query = em.createNativeQuery(<SQL Query which enforces the join order>);
| Operation op = (Operation)em.getSingleResult();
|

Thanks
Hoeft

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