There may be good reasons for explicitly qualifying all column references, but performance is not one of them, at least under Oracle 8i. I've just done some tests and there is absolutely no difference in the number of dictionary cache gets required during the parse, and no measurable difference in CPU usage.

I too remember being taught this back in version 6 days, and it is in the Gurry and Corrigan "Oracle Performance Tuning" book (2nd edition, page 138) so there may have been some validity to it in the past.

At a course some years ago, we were told that in join statements, we should qualify ALL our column names with the appropriate table name, not just those that may be ambiguous. The reason was that the parser would not need to spend time checking multiple tables to determine the table to which each column belongs. Is this still a valid rule?