Hibernate version: 2.1

Name and version of the database you are using:PostgreSQL

This is a general question. The Hibernate online reference doc says "Immutable classes, mutable="false", may not be updated or deleted by the application. This allows Hibernate to make some minor performance optimizations". I just wonder what optimization is implemented? And how much approximately improvement it can bring? Any figures or numbers to show the improvement?

In a simple testcase, where I create and persist an object, restart the session and then retrieved it, I did not see any difference in the SQL when using immutable and mutable objects. Could anyone kindly provide any elaboration on that?

Thanks,
Stan

AFAIK - You won't find any differences in the generated SQL...

The difference can appear when you have a large amount of such entities in the Hibernate session. Because they are declared 'immutable', Hibernate won't inspect them at flush time to determine if it has something to commit to the database or not.