While reading the book Expert Oracle Database Architecture, I noticed a very important bit of information about the DUAL table. In 10g, Oracle does not do a full table scan on DUAL if it does not need to. This is not the case in versions prior to 10g. To illustrate:

On the Oracle-l mailing list, Jared Still posted a message about a little experiment he did which concluded that Oracle (10gR1) always makes sure that DUAL returns one and only one row even if it contains more than one row.

I have repeated the experiment on my Oracle 10g XE database (Important: Do not do this [...]