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How to use ScrollableResults for Hibernate Queries when joining many different entities

I am using Spring Boot endpoints to return results from database queries. It works fine when using getResultList() on the TypedQuery. However I know I will have to managed very large data sets. I am looking into using ScrollableResults via hibernate but I cannot figure out how to actually reference the contents of each row.
StatelessSession session = ((Session) entityManager.getDelegate()).getSessionFactory().openStatelessSession();
criteriaQuery.multiselect(selections);
criteriaQuery.where(predicates.toArray(new Predicate[]{}));
Query<?> query = session.createQuery(criteriaQuery);
query.setMaxResults(5);
query.setFetchSize(1000);
query.setReadOnly(true);
ScrollableResults results = query.scroll(ScrollMode.FORWARD_ONLY);
while(results.next()){
Object row = results.get();
}
results.close();
session.close();
I have tried results.get(0), results.get(0)[0], results.getLong(0), Object[] row vs Object row, etc. With and without toString() on all of the options. Nothing I do gets more out of the row than the java object reference. I've tried casting as well and get a "cannot cast error". Sometimes I get an error, "query specifies a holder class". Not sure what that means because my criteria query is built by joining 1 or more entities where the entities and selected columns are not known before hand. So I am not actually specifying a class. They entities and selects are specified by user input. Any thoughts? Thanks!
UPDATE:
I can do System.out.println(scroll.getType(0)); and in this case observe a long.
But when I try to save that long (.getLong(0)) I get the error, "query specifies a holder class". Or again the cannot cast error.

Got it figured out. queryDetails is a CriteriaQuery<Tuple>
StatelessSession session = entityManagers.get("DatasourceName").unwrap(Session.class).getSessionFactory().openStatelessSession();
Stream<Tuple> resultStream = session.createQuery(queryDetails)
.setReadOnly(true)
.setMaxResults(100)
.setFetchSize(1000)
.setCacheable(false)
.getResultStream();
Iterator<Tuple> itr = resultStream.iterator();
while (itr.hasNext()){
//Get the next row:
Tuple row = itr.next();
}

A CriteriaQuery that uses multiselect produces an Object[] or javax.persistence.Tuple as result type. Maybe you should try debugging to see the actual object type and from there you can work further.
If you are processing and returning all rows anyway, there is no need to use the ScrollableResults API as you will have to create objects for all rows anyway. If your use case is to do some kind of aggregation, I would recommend you use an aggregate function instead and let the database do the aggregation.

GQL query where field is not null

I have an entity kind called Account. One of the fields is a String named selfie, which is basically the url to a selfie uploaded by a user. I want to fetch for users who have a selfie (so if a user does not have a selfie they should not be included in the result set). I have the following query. but it won't work because I have "NULL" as a string. What is the correct way for doing this? Again, I only want users who have selfies.
Filter selfie = new FilterPredicate("selfie", FilterOperator.GREATER_THAN, "NULL");
Query query = new Query(Account.class.getSimpleName()).setFilter(selfie);
FetchOptions options = FetchOptions.Builder.withLimit(30);
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
QueryResultList<Entity> entities = datastore.prepare(query).asQueryResultList(options);
Also I am open to a JDO/JPA way of doing this on App-Engine (but it must work on App-Engine).

After some trial and error on https://console.cloud.google.com/datastore/entities/query/gql
I found this work around expression for IS NOT NULL:
> NULL
Example:
Select * from State where customer_id > NULL
I haven't tested this on a field with negative values.

This should be as simple as
Filter selfie = new FilterPredicate("selfie", FilterOperator.NOT_EQUAL, null);
where you actually pass the Java null value (not a String) and filter with not equals (see https://stackoverflow.com/a/22200137).
Personally, I've been using JDO for my datastore management, which has its pros and cons. Let me know if you'd like to see a solution using JDO as well. Cheers!

How to retrieve only the PKs of a table

I'm working with Java Apache Cayenne, under a MySQL DB.
I have quite a large table with a single bigint PK and some fields.
I'd like to retrieve just only the PK values and not all the object that maps this entity, as it would be too resource-consuming.
Is there a snippet that I can use, instead of this one that retrieves all the objects?
ObjectContext context = ...
SelectQuery select = new SelectQuery(MyClass.class);
List<MyClass> result = context.performQuery(select);

You should try using SQLTemplate instead of SelectQuery.
Here's a quick example:
ObjectContext context = ...
SQLTemplate select = new SQLTemplate(MyClass.class, "SELECT #result('PK_COLUMN' 'long') FROM MY_TABLE");
List result = context.performQuery(select);
You can find more information here

+1 for Josemando's answer. And here is another way that may work in case you are planning to only work with a subset of fetched objects:
ObjectContext context = ...
SelectQuery select = new SelectQuery(MyClass.class);
select.setPageSize(100);
List<MyClass> result = context.performQuery(select);
'setPageSize' ensures that 'result' only contains ids, until you attempt to read an object from the list. And when you do, it will resolve it page-by-page (100 objects at a time in the example above). This may fit a number of scenarios. Of course if you iterate through the entire list, eventually all objects will be fully resolved, and there will be no memory benefit.

Order by another table with Liferay's DynamicQuery

I have a problem while I'm making a Dynamic Query in Liferay 6. I'm trying to make a query to order JournalArticles based on their view count. The view count is specified in another table (AssetEntry).
I'm stuck with this:
DynamicQuery query = DynamicQueryFactoryUtil.forClass(
JournalArticle.class, "articleParent", PortalClassLoaderUtil.getClassLoader());
//adding criterions
query.add(...);
DynamicQuery dq0 = DynamicQueryFactoryUtil.forClass(AssetEntry.class, "asset",
PortalClassLoaderUtil.getClassLoader())
.setProjection(ProjectionFactoryUtil.property("asset.classPK"))
.add(PropertyFactoryUtil.forName("asset.companyId")
.eqProperty("articleParent.companyId"))
.add(PropertyFactoryUtil.forName("asset.groupId")
.eqProperty("articleParent.groupId"));
query.add(PropertyFactoryUtil.forName("articleParent.resourcePrimKey").in(dq0))
.addOrder(OrderFactoryUtil.desc("asset.viewCount"));
With this I get an error message saying: could not resolve property: asset of: com.liferay.portlet.journal.model.impl.JournalArticleImpl.
If I remove the addOrder-call, this error disappears. How should I add the order statement so the main query is aware of asset.viewCount?

AssetEntryQuery assetEntryQuery = new AssetEntryQuery();
assetEntryQuery.setClassName(JournalArticle.class.getName());
assetEntryQuery.setXXX //adding criterions
assetEntryQuery.setOrderByCol1("viewCount");
List<AssetEntry> assetEntries = AssetEntryServiceUtil.getEntries(assetEntryQuery);

I am afraid that there is no direct way to do this with the DynamicQuery API.
I think you would need to use Service builder Finders i.e. Custom Query with Service builder.

You can't use dynamic query because there is no direct reference from JournalArticle entity to AssetEntry entity.
One possibility is to retrieve ordered ids of articles from the AssetEntry table (basically you dq0), then do another query and do the sorting programatically (you have the ids ordered).
Finally I think that this line
query.add(PropertyFactoryUtil.forName("articleParent.resourcePrimKey").in(dq0))
doesn't do what you think it does. resoucePrimKey is reference to resource table for permissions. You need to use id column.

Loading multiple entities by id efficiently in Hibernate

So, I'm getting a number of instances of a particular entity by id:
for(Integer songId:songGroup.getSongIds()) {
session = HibernateUtil.getSession();
Song song = (Song) session.get(Song.class,id);
processSong(song);
}
This generates a SQL query for each id, so it occurred to me that I should do this in one, but I couldn't find a way to get multiple entities in one call except by running a query. So I wrote a query
return (List) session.createCriteria(Song.class)
.add(Restrictions.in("id",ids)).list();
But, if I enable 2nd level caching doesn't that mean that my old method would be able to return the objects from the 2nd level cache (if they had been requested before) but my query would always go to the database.
What the correct way to do this?

What you're asking to do here is for Hibernate to do special case handling for your Criteria, which is kind of a lot to ask.
You'll have to do it yourself, but it's not hard. Using SessionFactory.getCache(), you can get a reference to the actual storage for cached objects. Do something like the following:
for (Long id : allRequiredIds) {
if (!sessionFactory.getCache().containsEntity(Song.class, id)) {
idsToQueryDatabaseFor.add(id)
} else {
songs.add(session.get(Song.class, id));
}
}
List<Song> fetchedSongs = session.createCriteria(Song.class).add(Restrictions.in("id",idsToQueryDatabaseFor).list();
songs.addAll(fetchedSongs);
Then the Songs from the cache get retrieved from there, and the ones that are not get pulled with a single select.

If you know that the IDs exist, you can use load(..) to create a proxy without actually hitting the DB:
Return the persistent instance of the given entity class with the given identifier, obtaining the specified lock mode, assuming the instance exists.
List<Song> list = new ArrayList<>(ids.size());
for (Integer id : ids)
list.add(session.load(Song.class, id, LockOptions.NONE));
Once you access a non-identifier accessor, Hibernate will check the caches and fallback to DB if needed, using batch-fetching if configured.
If the ID doesn't exists, a ObjectNotFoundException will occur once the object is loaded. This might be somewhere in your code where you wouldn't really expect an exception - you're using a simple accessor in the end. So either be 100% sure the ID exists or at least force a ObjectNotFoundException early where you'd expect it, e.g. right after populating the list.

There is a difference between hibernate 2nd level cache to hibernate query cache.
The following link explains it really well: http://www.javalobby.org/java/forums/t48846.html
In a nutshell,
If you are using the same query many times with the same parameters then you can reduce database hits using a combination of both.

Another thing that you could do is to sort the list of ids, and identify subsequences of consecutive ids and then query each of those subsequences in a single query. For example, given List<Long> ids, do the following (assuming that you have a Pair class in Java):
List<Pair> pairs=new LinkedList<Pair>();
List<Object> results=new LinkedList<Object>();
Collections.sort(ids);
Iterator<Long> it=ids.iterator();
Long previous=-1L;
Long sequence_start=-1L;
while (it.hasNext()){
Long next=it.next();
if (next>previous+1) {
pairs.add(new Pair(sequence_start, previous));
sequence_start=next;
}
previous=next;
}
pairs.add(new Pair(sequence_start, previous));
for (Pair pair : pairs){
Query query=session.createQuery("from Person p where p.id>=:start_id and p.id<=:end_id");
query.setLong("start_id", pair.getStart());
query.setLong("end_id", pair.getEnd());
results.addAll((List<Object>)query.list());
}

Fetching each entity one by one in a loop can lead to N+1 query issues.
Therefore, it's much more efficient to fetch all entities at once and do the processing afterward.
Now, in your proposed solution, you were using the legacy Hibernate Criteria, but since it's been deprecated since Hibernate 4 and will probably be removed in Hibernate 6, so it's better to use one of the following alternatives.
JPQL
You can use a JPQL query like the following one:
List<Song> songs = entityManager
.createQuery(
"select s " +
"from Song s " +
"where s.id in (:ids)", Song.class)
.setParameter("ids", songGroup.getSongIds())
.getResultList();
Criteria API
If you want to build the query dynamically, then you can use a Criteria API query:
CriteriaBuilder builder = entityManager.getCriteriaBuilder();
CriteriaQuery<Song> query = builder.createQuery(Song.class);
ParameterExpression<List> ids = builder.parameter(List.class);
Root<Song> root = query
.from(Song.class);
query
.where(
root.get("id").in(
ids
)
);
List<Song> songs = entityManager
.createQuery(query)
.setParameter(ids, songGroup.getSongIds())
.getResultList();
Hibernate-specific multiLoad
List<Song> songs = entityManager
.unwrap(Session.class)
.byMultipleIds(Song.class)
.multiLoad(songGroup.getSongIds());
Now, the JPQL and Criteria API can benefit from the hibernate.query.in_clause_parameter_padding optimization as well, which allows you to increase the SQL statement caching mechanism.
For more details about loading multiple entities by their identifier, check out this article.