Terracotta's tech lead Alex Miller summarizes a few months of Hibernate performance tuning. A few common things were encountered and in this article he talks about Hibernate query caching.

Alex gives a good overview of the Hibernate Caching, as Hibernate has several levels and kinds of cache. A first level cache of Hibernate specifically to a particular open session. But there is also a second level cache that is shared across session and which contains four types:

•  Entity
• Collection
• Query
• Timestamp

The entity and collection regions cache date from entities and their relations, but not the entities themselves. The query and timestamp cache are related and an overview and discussion of the issues involved can be read in his very interesting article.