Full joins are sub-optimal in ORCA as they are implemented as a UNION of
Left Outer Join AND Left Anti-Semi Join. However, GPDB provides a full
outer join operator. Therefore, until ORCA implements a more optimal
FULL JOIN, it will fall back to the Postgres legacy query optimizer for
queries with FULL JOINs.

Co-authored by: Sambitesh Dash sdash@pivotal.io
Co-authored by: Ashuka Xue axue@pivotal.io