What is the performance implication of using HQL aggregate query vis-a-vis native SQL aggregate query ? For example if I am to get the average age of large population stored in database. Lets assume no of record is large.

My understanding is that HQL will fetch all people objects in client and compute the average age there. Whereas, in case of native SQL the computation will be done by sql AVG in the database and only the result will be returned to the client.

Wouldn't that have some performance impact ?

I think that anything the HQL parser doesn't recognize gets passed through to the SQL statement, so any function you want to use will work just fine. But it's very easy to turn on SQL logging and see what the HQL query turns into. Or if you use Eclipse and have the tools installed, you can see what the resulting SQL will be in the plugin.