How to Improve performance of iteration loops

I have a java method which finds all children (leaves) of a parent. It works ok.but there is performance issue. How can i make it work faster? When the children gets large it becomes problematic. Here is the method:

Private void getChildren(String orgId, List<String> children, List<String> organs, int j){
Query query= entityManager.createNativeQuery("select c.organization_id from organization c where c.parent_org_id="+orgId);
if(query.getResultList().size()>0)
  organs.addAll((List<String>) query.getResultList());
else
  children.add(orgId);
for(int i=j+1; i<organs.size();i++){
j=i;
query=entityManager.createNativeQuery("select c.organization_id from organization c where c.parent_org_id="+String.valueOf(organs.get(i)));
if(query.getResultList().size()>0)
  organs.addAll((List<String>) query.getResultList());
else
  children.add(String.valueOf(organs.get(i)));
}
}

I call this method like

getChildren("68",new ArrayList<>(),new ArrayList<>(),-1)

I have oracle as database so if it is better to acheive this via oracle query please note me.
By the way my spring version doesn't support streams!

You basically have a 1+N select problem (issue a single query for each id in your collection). Don't do this, instead use the IN clause and issue a single query!.

Another thing you shouldn't be doing is use concat for your query creation, never do this! What you should do is just return the result and use a query with an IN clause.

String query = "select c.organization_id from organization c where c.parent_org_id IN (:ids))
return em.createNativeQuery(query).setParameters("ids", organs);

or even better use a single query altogether. You can use a self join for that)

String query = "SELECT c1.organization_id " +
               "FROM organization c1 LEFT OUTER JOIN organization c2 " +
               "ON c1.parent_org_id=c2.organization_id " +
               "WHERE c2.parent_org_id=?"; 
return em.createNativeQuery(query, String.class).setParameter(1, orgId).getResultList();

Judging from your solution I would strongly recommend studying SQL and JPA.

Try it in a big query

select c.organization_id from organization, (
select c.organization_id as sub1 from organization c where c.parent_org_id="+orgId +") as x_ where organization.parent_org_id = x_.sub1

The reason why it is slow, is that you query the database for every child. This is very inefficient, I assume even the load on the database will be smaller.

Answer provided by Deinum only gets the direct children. Whereas my java method returns all children including nested ones.

I have found a query for that which is much more optimized than my java method. It uses temporary tables:

with temp_tbl (id,parent_id) as 
(
select organization_id,parent_org_id from organization where parent_org_id =? 
union all 
select c.organization_id ,c.parent_org_id from organization c join temp_tbl t on t.id=c.parent_org_id and t.parent_id !=t.id
)
Select to_char(g.id) from temp_tbl g 

At the end i return to_char(g.id) to make it a string instead of big decimal. Because em.createNativeQuery(query, String.class) didn't turn it to string and threw error for me.
So just use em.createNativeQuery(query)