Index Fast Full Scan NOT used when using other columns than in the index

Hi,

i'vh currently the  problem hat my table has 170GB (50million rows) size and approx 25 columns

i'vh one index (22GB size) with three columns of the table (in this order):

  - processing_status  (not null)

  - billing_type (nullable)

  - aggegration_id (nullable)

The final result set is about 20k-50k rows

Because of the low selectivity of all columns in the where clause, I want to use an Fast Full Index Scan to filter the data and than continue to table. The cardinality after index filte should be ok for table access.

So here my Problem:

If I'm goint to use only a count(*) or the three columns in the INDEX i'll get the following plan: