Hello, Mimai:
That looks like an interesting question. I could not find that text in the JPA tutorial. Could this be from a book, such as "Pro JPA"?
Either way, the warning does not appear to be about failure to modify something you intend to. Instead, it appears to be a warning about modifying things that you do not intend to. I am going out on a limb slightly, here, because I have not explored this topic. But, I think they are warning us that if we use native SQL to retrieve entities into the EntityManager, the EM won't be "smart enough" to populate things we failed to mention in our query. If you want an example, you might try
this, which leaves out the salary as well. Then try persisting that entity again, and see if Salary becomes a default or null value.
Please let me know how it goes. I am quite curious. Sorry if this is a goose chase, but at least it doesn't take much to try it.
BTW, I posted because I have gotten curious about JPA. I tried it out about a year and a half ago, and used the Criteria API. I recently heard that it can be used for some NoSQL databases, too.
I hope it helps.