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JPA - One to Many Mapping using Native Query

I've a SQL Query which returns one to many mapping data, for example I've JOB table along with multiple one to one dependent tables and a Warning Table with one-many mapping.
JOB 1 can have n number of warnings to it.
Right now we are handling it from Java logic but Is it possible to achieve it using JPA Native Query?
I see the same scenario's mentioned before How to map native sql results to oneToMany field with SqlResultSetMapping in JPA
This is the exact thing I'm looking for ? Will the Entity take care when used with native Query?

Fetch efficiently huge number of entities from DB meeting criteria

Let's say I have about half of million records in table A. Of course table A is joined with table B, C, etc. Now I have to fetch entities from table A which meet my criteria. My criteria consists of about 20-30 rules e.g. name in table A has to be like 'something' or date from table B for joined record from table A with ID=1 should be earlier than today. I see three solutions:
Write native query and put parameters from my criteria. But in this case I will join so many tables that it doesn't seem to me as a good example.
Fetch all records from table A and then check in Java every rule for each record. But this seems for me as the worst possible solution.
Use JPA Criteria. But to be honest I do not know how efficient they are while there are so many records and so many joined tables. What's more it seems to me like working with Criteria can be a little irritating when I have so many rules to match.
Maybe there is another (better) solution to my problem but I cannot see it now. I need to add that I need these fetched entities stored in Java collection because when they are all fetched then I have to work with them (e.g. generate report or create some updates in DB basing on these information).
I hope I described my problem clear and I will be thankful for every tip how to optimize such query.

Retrieve Hibernate associated entity with pagination

I am stuck with an issue. I have 3 tables that are associated with a table in one to many relationship.
An employee may have one or more degrees.
An employee may have one or more departments in past
An employee may have one or more Jobs
I am trying to fetch results using named query in a way that I fetch all the results from Degree table and Department table, but only 5 results from Jobs table. Because I want to apply pagination on Jobs table.
But, all these entities are in User tables as a set. Secondly, I don't want to change mapping file because of other usages of same files and due to some architectural restrictions.
Else in case of mapping I could use BatchSize annotation in mapping file, which I am not willing to do.

The best approach is to write three queries:
userRepository.getDegrees(userId);
userRepository.getDepartments(userId);
userRepository.getJobs(userId, pageIndex);
Spring Data is very useful for pagination, as well as simplifying your data access code.
Hibernate cannot fetch multiple Lists in a single query, and even for Sets, you don't want to run a Cartesian Product. So use queries instead of a single JPQL query.

Hibernate performance issue: query execution extremely slow

The following hibernate query is being used to fetch a list of ProductCatalogue records, by passing in catId and inventoryId
select prodcat from ProductCatalogue prodcat where prodcat.prodSec.prodId=:catId and prodcat.prodPlacedOrder.inventoryId=:inventoryId
The tables ProductCatalogue and ProdPlacedOrder are tables with 3 lakh + records. inventoryId is a column in prodOrder table, and prodPlacedOrder extends prodOrder table.
This query on execution takes a lot of time, and the single hibernate query shoots off many complex sql queries.
Any suggestions on what might be the issue and how to modify it such that the query is executed faster?

Difficult to say without more info but try making ProdPlacedOrder as LAZY fetch if you dont need any data from that table.
Also as phatmanace, mentioned - check your indices.

What method of joining entities is faster?

Here are two queries retrieving equivalent data:
SELECT DISTINCT packStatus
FROM PackStatus packStatus JOIN FETCH packStatus.vars, ProcessEntity requestingProcess
WHERE
packStatus.status.code='OGVrquestExec'
AND packStatus.procId=requestingProcess.activityRequestersProcessId
AND requestingProcess.id='1000323733_GU_OGVProc'
SELECT DISTINCT packStatus
FROM PackStatus packStatus JOIN FETCH packStatus.vars
WHERE
packStatus.status.code='OGVrquestExec'
AND packStatus.procId=(SELECT requestingProcess.activityRequestersProcessId FROM ProcessEntity requestingProcess WHERE requestingProcess.id='1000323733_GU_OGVProc')
These queries differ in the method how requstingProcess is joined with packStatus. In general, which of these two methods is more preferable in terms of performance? I'm using JPA 1.2 provided by Hibernate 3.3 on Postgres 8.4.
UPD: I've replaced fake queries with real queries from my app. Here is SQL generated by Hibernate for first and second query. Links to query plans: first, second. Query plans look pretty the same. The only difference is what moment data from bpms_process table is aggregated to query result at. But I don't know is it right to generalize these results? Would query plans be almost the same for queries differing only in joining method? Is it possible to get a big difference in query cost by changing joining method?

Use EXPLAIN ANALYZE and see.
I won't be surprised if they get turned into the same query plan.
See: https://stackoverflow.com/tags/postgresql-performance/info