Lukas Eder talks about implicit join elimination:
One of jOOQ’s key features so far has always been to render pretty much exactly the SQL that users expect, without any surprises – unless some emulation is required to make a query work, of course. This means that while join elimination is a powerful feature of many RDBMS, it isn’t part of jOOQ’s feature set, so far.
As Lukas mentions, many relational database products already do this–SQL Server is an example of one product that does. But not all of them do, so it’s nice to have that option available in the data access layer.