You have helped me previously setting up skinny indexing here: QUESTION_ID:9542855000346335758

This has been implemented, and is now in production and working really well and is super fast; thanks for all the previous help with this.

I have a follow up question. Although most of the orders tables have millions of rows, unprocessed orders are generally only a few thousand rows, and so only hitting skinny indexes by indexing virtual columns only holding data for unprocessed orders as you explained in the question above has worked brilliantly. In most cases, a FAST FULL SCAN is performed on the indexes as they are relatively small, and we select most of the data held in those indexes.
What I find odd is when I look at the explain plan in SQL Developer, if I add a predicate to the query, it's not shown in the explain plan. The explain plan remains the same i.e. just a FAST FULL SCAN. If I don't see the predicate in the explain plan, where is that being processed? I know a FAST FULL SCAN grabs all the data from the index in one go. Would I not still see the predicate in the explain plan. I can send screen shots from sql dev if that helps.
Thanks.

set serveroutput off
select * from t
where  t_id = 1;

select * 
from   dbms_xplan.display_cursor( format => 'BASIC +PREDICATE');

---------------------------------------------------    
| Id  | Operation                   | Name        |    
---------------------------------------------------    
|   0 | SELECT STATEMENT            |             |    
|   1 |  TABLE ACCESS BY INDEX ROWID| T           |    
|*  2 |   INDEX UNIQUE SCAN         | SYS_C008715 |    
---------------------------------------------------    
                                                       
Predicate Information (identified by operation id):    
---------------------------------------------------    
                                                       
   2 - access("T_ID"=1)