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
