As part of optimization, we can simplify queries by merging a derived
table into it's parent instead of materializing it. When this happens
the context paths, describing how each SELECT_LEX is positioned in our
query needs updating. The best place to do this is in the call to
SELECT_LEX::exclude_level().
We follow MySQLs example here.
Approved by Sanja Byelkin (sanja@mariadb.com)
