[FIXED] Effective scroll in hibernate: session.evict vs session.clear vs CacheMode.IGNORE

Issue

Consider a typical task, when we have to build huge file report, by scrolling appropriate results from DB. ORM framework is Hibernate. We know, that there are 3 ways, how to avoid OutOfMemoryException for such pattern:

1. Use session.evict(...):

   ScrollableResults customers = session.createQuery("from Customers order by id").scroll(ScrollMode.FORWARD_ONLY);
   while (customers.next()) {
       Customer customer = (Customer) customers.get(0);
       addDataToReport(customer);
       session.evict(customer);
   }
   

2. Use session.clear():

   ScrollableResults customers = session.createQuery("from Customers order by id").scroll(ScrollMode.FORWARD_ONLY);
   int i = 0;
   while (customers.next()) {
       Customer customer = (Customer) customers.get(0);
       addDataToReport(customer);
       if ( ++i % 1000 == 0) session.clear();
   }
   

3. Use CacheMode.IGNORE:

   ScrollableResults customers = session.createQuery("from Customers order by id").setCacheMode(CacheMode.IGNORE).scroll(ScrollMode.FORWARD_ONLY);
   while (customers.next()) {
       Customer customer = (Customer) customers.get(0);
       addDataToReport(customer);
   }
   

So, the question is: which of these methods is most better (in sense of performance) for mentioned purposes? Or may be there are more effective other methods?

Solution

• CacheMode.IGNORE has nothing to do with your problem. It's about 2nd level cache, not session cache.
• evict() doesn't evict related entities if relationship is configured without CascadeType.DETACH or CascadeType.ALL. It means that:
  1. Related entities without cascaded eviction will accumulate in memory
  2. Related entities with cascaded eviction will be reloaded on each iteration, even if the are the same
• Threfore approach with clear() is the best option:
  1. When invoked, it evicts all entities from the session cache
  2. It's not invoked on each iteration, so that you can leverage session cache for related entities

Answered By - axtavt
Answer Checked By - Mary Flores (JavaFixing Volunteer)