Did you compare the execution plans of both queries? Without doing that, it's hard to know why the planner did what it did. Maybe statistics were off and it was

Did you compare the execution plans of both queries? Without doing that, it's hard to know why the planner did what it did. Maybe statistics were off and it was choosing a worse plan for the left join because it thought it'd be faster, where the sub-select it was forced into what should have been an inferior plan (if statistics were correct), which in reality was faster.