If nothing is entered in the topic field, I got no search results eventhough a user id was put in the advanced options field.
Yep. That's a limitation. If you need to list all threads started by some user, then you would have to go to her profile and click on the topic list link.
Search is performed using a separate index (not he main database), so simply iterating over threads without any keywords wouldn't work.
dsb38327. (the word) All (in the topic field) shows all of the post I have created.
Actually, that's not the case. It might look that way because the word "all" is rather common.
So chances are a lot of threads contain it at least once.
When you limit the listing to just one particular user and search for a common word, it might appear like all threads are returned. But if there is a thread that doesn't contain that specific word, you won't see it listed. At least you shouldn't.
If you actually find such example, then please let me know. I would be interested in seeing it.
There is a list of stop words that don't get indexed, such as "the". But "all" is not a stop word, yet it's used frequently enough to cause what you are seeing.