Sometimes the most interesting findings start with a simple question. After publishing my recent post on Oracle 26ai JOIN TO ONE, Iudith Mentzel pointed out something in one of the query transformations that looked perfectly logical – but turned out not to describe what Oracle actually does.

A small test led me one level deeper, from DBMS_UTILITY.EXPAND_SQL_TEXT to the CBO trace, and revealed another interesting aspect of JOIN TO ONE: the way Oracle handles its implicit LEFT OUTER JOIN.