optimizing large selects in hibernate/jpa with 2nd level cache

Question

I have a user object represented in JPA which has specific sub-types. Eg, think of User and then a subclass Admin, and another subclass Power User.

Let's say I have 100k users. I have successfully implemented the second level cache using Ehcache in order to increase performance and have validated that it's working. http://docs.jboss.org/hibernate/core/3.3/reference/en/html/performance.html#performance-cache

I know it does work (ie, you load the object from the cache rather than invoke an sql query) when you call the load method. I've verified this via logging at the hibernate level and also verifying that it's quicker.

However, I actually want to select a subset of all the users...for example, let's say I want to do a count of how many Power Users there are.

Furthermore, my users have an associated ZipCode object...the ZipCode objects are also second level cached...what I'd like to do is actually be able to ask queries like...how many Power Users do i have in New York state...

However, my question is...how do i write a query to do this that will hit the second level cache and not the database. Note that my second level cache is configured to be read/write...so as new users are added to the system they should automatically be added to the cache...also...note that I have investigated the Query cache briefly but I'm not sure it's applicable as this is for queries that are run multiple times...my problem is more a case of...the data should be in the second level cache anyway so what do I have to do so that the database doesn't get hit when I write my query.

cheers, Brian

Answer 1

(...) the data should be in the second level cache anyway so what do I have to do so that the database doesn't get hit when I write my query.

If the entities returned by your query are cached, have a look at Query#iterate(). This will trigger a first query to retrieve a list of IDs and then subsequent queries for each ID... that would hit the L2 cache.