Hibernate and sets – use at your peril
Posted by Chris Clark on December 6, 2011
For the past 3 weeks I’ve spent hours tuning the performance in one of the applications we use. Unfortunately rather than spend some time and come up with a proper reporting solution I was tasked to fix the existing solution which is to report off our live transactional system. We use EJB3 on JBoss so hibernate it is, which I personally think is an excellent framework for getting your application up and running quickly. Hibernate have always stated that performance isn’t it’s goal so I can’t blame hibernate for anything I’ve found. I tackled the problem in the usual way, add some sql logging and see what’s really happening and look for the usual suspects, lazy loading, code structure etc. Where I saw collections being loaded up separately I added Join fetches and rerun the reports. This worked really well then I noticed that no matter how I wrote the JPQL it always seemed to be running seperate queries for some of the realtionships. That’s when the bulb flashed they were defined as Sets!!
An example of such a thing:
@OneToMany(mappedBy="transaction") private Set taxComponents = new HashSet(1);
I understand why sets are used but as I’m in control of what gets put in I can safely use:
@OneToMany(mappedBy="transaction") private Collection taxComponents = new ArrayList(1);
So using set’s is great but it comes at a cost and beware it’s not just fetches but deletes and inserts have a similar issue in that they get deleted one by one and then reinserted to ensure uniqueness, so if you do update the set it also has a performance hit.
The performance of the major report moved from 30 mins to 6 mins, still not lightning fast but a good improvement. I could of course move to native SQL but I didn’t want to have to rewrite a load of code to do that.