transform a Full outer join to left outer join

[Blacksails]

Hi, i have a 'problem' with some business report requirements and the model that i have made.
.my model consist of various namespace with fact table and dimensions.we have some shared dimensions.now on my list report that consist on  measures of two different fact tables A e B and fields of conformed dimensions C.when i look.at sql (native and cognos are identical,we have netezza appliance)it returns full outer joins stitching on common fields..normal and correct way of drill accross.also bobi and other bi tools make this  way. Now my questions and doubts.                                                          1 question.   how i could  obtain a left outer join with one query on a fact table A driving the sql?! it is possible to remove full outer join in framework.manager or Rstudio? Governor  settings?                                     2 question .   why cognos when creates full join stitches on fields(descriptive fields) and not on the pks of my shared dimensions? it is slower and dlesnt use indexed pk.Right?                                       3 question. i have obtained i report studio  a left outer join putting a filter on the query list and setting  A not null filter with timing  after auto aggregation on a field of my fact table A. Is correct this workaround?on native sql is present as as a subquery with a where clause.is done locally or on db?  thanks everybody for your attention and your help. ps sorry for my english