Hibernate and efficient queries
First post of the year (bad boy)...
I've spent a lot of this year so far jumping between lots of different things. I've started dipping into the new SiteBuilder code which is far more familiar as it is now Spring/Hibernate based rather than Struts/EJB.
I've also as usual been working on Single Sign-On and BlogBuilder.
As the complexity of BlogBuilder grows and our page views grows (now averaging more than 50,000 proper real people page views per day), it has become more and more important to optimise BlogBuilder for better performance.
Hongfeng our resident Oracle expert pointed me in the direction of quite a lot of particularly bad and slow pieces of SQL that were being generated out of BlogBuilder. The problem with BlogBuilder is that it is very very dynamic. We do not serve any static pages as every single page is customised to the currently logged in user as every blog/image/entry has its own permissions. There are also just a lot of different views on the blogs data; daily views, monthly views, favourites views, entries by tags, blogs by group, images by day, etc…
When using Hibernate 2 I did most of these queries with HQL, and it worked quite well, but I'm starting to feel the strain as some of the queries got more and more complicated.
With Hibernate 3 I can now take advantage of the Criteria API, which is quite nice for building complicated queries, but it still has some problems so I've now got a mix of HQL, Criteria and plain old SQL when a particularly complicated aggregation is needed.
Don't forget to turn on query caching and specifically tell your criteria and queries to cache as although the documentation says that for most queries caching doesn't make much difference, I've found it can make a huge difference.
Another little trick is to be careful with date range queries. If you want to do something like find items based on the current time, round your time to the nearest hour or minute rather than passing in a date with second or millisecond accuracy as this will prevent those queries being cached for more than a second…not a lot of good.
Another trick when moving from Hibernate 2 to Hibernate 3 is that you used to have to do "query.iterate().next()" to get a result when you knew there was just a single result (such as a count query), but now there is the uniqueResult() method. It is important to switch over because the uniqueResult() calls get cached, but the iternate().next() ones don't.