left join RDB$RELATIONS R on R.RDB$SYSTEM_FLAG = RF.RDB$FIELD_POSITION
RF.RDB$RELATION_NAME BETWEEN 'RDB$RELATIONS' AND 'RDB$RELATIONS2'
PLAN JOIN (RF INDEX (RDB$INDEX_4), R NATURAL)
will be good to see here HASH like is done for inner joins
PLAN HASH (R NATURAL, RF INDEX (RDB$INDEX_4))