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How to implement Hierarchical queries in JPA?

I have a huge oracle database with large number of stored procedures which I need to convert in microservice. In one of those procedures there is a hierarchical query in the form of :-
Select column_names from table_name start with column_name in (subquery) connect by nocycle prior (condition)
I need to convert this functionality into java code as I don't know how to use this in JPA query and I also cannot use native query. As I understand, this sort of query returns data in a tree like structure. I need to maintain that same structure as I further want to join this to another view. I would like to know how can I implement such a functionality in java using simple JPA queries ?

Is there a way to make query return a ResultSet?

I have the following query:
#Select("SELECT* FROM "+MyData.TABLE_NAME+" where data_date = #{refDate}")
public List<MyData> getMyData(#Param("refDate") Date refDate);
This table data is HUGE! Loading so many rows in memory is not the best way!
Is it possible to have this same query return a resultset so that I can just iterate over one item?
edit:
I tried adding:
#ResultType(java.sql.ResultSet.class)
public ResultSet getMyData(#Param("refDate") Date refDate);
but it gives me:
nested exception is org.apache.ibatis.reflection.ReflectionException: Error instantiating interface java.sql.ResultSet with invalid types () or values (). Cause: java.lang.NoSuchMethodException: java.sql.ResultSet.<init>()

I'd suggest you use limit in your query. limit X, Y syntax is good for you. Try it.
If the table is huge, the query will become slower and slower. Then the best way to to iterate will be to filter based on id and use limit.
such as
select * from table where id>0 limit 100 and then
select * from table where id>100 limit 100 etc

There are multiple options you have ...
Use pagination on database side
I will just suppose the database is oracle. However other db vendors would also work. In oracle you have a rownum with which you can limit number of records to return. To return desired number of records you need to prepare a where clause using this rownum. Now, the question is how to supply a dynamic rownum in a query. This is where dynamic sqls of mybatis comes in use. You can pass these rownum values inside a parameter map which there onwards you can use in your query inside a mapper xml using a #{} syntax. With this approach you filter the records on db level itself and only bring or prepare java objects which are needed or in the current page.
Use pagination on mybatis side
Mybatis select method on sqlSession has a Rowbounds attribute. Populate this as per your needs and it will bring you those number of records only. Here, you are limiting number of records on mybatis side whereas in first approach the same was performed on db side which is better performant .
Use a Result handlers
Mybatis will give you control of actual jdbc result set. So, you can do/iterate over the result one by one here itself.
See this blog entry for more details.

Using Hibernate setFirstResult without any ordering

I am new to hibernate. I am confused with criteria's setFirstResult method.
From the documentation it seems hibernate returns rows from the the given number.
Since SQL query does not guarantee the ordering of rows without order by clause,
how setFirstQuery works in this case(without orderBy clause)?
Does hibernate read index information from the database?
If I execute same SQL query multiple times, ordering might change, in this case how setFirstResult work?

Hibernate can not do something by its own unless its supported by underlying databases. Because Hibernate queries finally get transformed to Sql only.
Having said that it uses underlying databases capabilities like for PostgresSQL and MySQL it will generate query like limit ? offset ? .
You can add custom order using addOrder
.addOrder( Order.asc("name") )

It is your task to add an order by, the function as you noticed won't guarantee same results if executed several times over the same set of data.
setFirstResult is typically used in pagination.

Deleting multiple rows in a one statement

I have some trivial table in database (let say Oracle10g) and I need to implement at DAO ability to delete multiple records. The method remove() receives as a parameter an array of ids (integers).
For now I have a query string "DELETE FROM news WHERE id = ?" which I use at PreparedStatement. I simply add batch for every id from array and then perform execute on PreparedStatement.
I wonder if there any ability to perform it through one query statement, something like "DELETE FROM news WHERE id IN ?". But I cannot find how to properly set an array of integers instead of '?'.
The same question applies to Hibernate and JPA. If there any constructions to solve this ? Because now I use batch-like-way: add Query to Session on every id from array and commit transaction.

The best I've seen done is to dynamically build the String used by the PreparedStatement, inserting the proper # of ?, sequences, then use a for loop to call setInt as appropriate for each row - each row to be deleted in your case.
JPA provides a special syntax for this (can accept a Collection to populate the list of arguments), since it has to create the SQL anyway - and likely does so very similar to how I just described. Specifics as to the API calls (for both JPA and HQL) are available at Hibernate HQL Query : How to set a Collection as a named parameter of a Query? .

Hibernate Criteria Limit mechanism?

Hibernate Criteria support provides a setMaxResults() method to limit the results returned from the db.
I can't find any answer to this in their documentation - how is this implemented? Is it querying for the entire result set and then returning only the request number? Or is it truly limiting the query on the database end (think LIMIT keyword as in mySql).
This is important because if a query could potentially return many many results, I really need to know if the setMaxResults() will still query for all the rows in the database (which would be bad).
Also - if its truly limiting the number of rows on the database end, how is it achieving this cross-db (since I don't think every rdbms supports a LIMIT functionality like mySql does).

Hibernate asks the database to limit the results returned by the query. It does this via the dialect, which uses whatever database-specific mechanism there is to do this (so for SQL Server it will do somthing like "select top n * from table", Oracle will do "select * from table where rownum < n", MySQL will do "select * from table limit n" etc). Then it just returns what the database returns.

The class org.hibernate.dialect.Dialect contains a method called supportsLimit(). If dialect subclasses override this method, they can implement row limit handling in a fashion native to their database flavor. You can see where this code is called from in the class org.hibernate.loader.Loader which has a method titled prepareQueryStatement, just search for the word limit.
However, if the dialect does not support this feature, there is a hard check in place against the ResultSet iterator that ensures Java object (entity) results will stop being constructed when the limit is reached. This code is also located in Loader as well.

I use both Hibernate and Hibernate Search and without looking at the underlying implementation I can tell you that they definitely do not return all results. I have implemented the same query returning all results and then changed it to set the first result and max results (to implement pagination) and the performance gains were massive.
They likely use dialect specific SQL for this, e.g. LIMIT in MySQL, ROWNUM in Oracle. Your entity manager is aware of the dialect that you are using so this is simple.
Lastly if you really want to check what SQL Hibernate is producing for this query, just set the "show_sql" property to true when you create your entity manager / factory and it spits out all the SQL it is running to the console.

HQL does not suppport a limitation inside a query like in SQL, only the setMaxResults() which you also found.
To find out if it transform the setMaxResults() into a LIMIT query, you can turn on your SQL logging.

I know Question is bit old. But yes setMaxResults() is truly limiting the number of rows on the database end.
If you really look into your Hibernate SQL output, you can find the following SQL statement has been appended to your query.
limit ?