1. Чтобы была возможность использовать Index Full Scan / Index Fast Full Scan 
для "select a,b,c from tab where a=:1 and b=:2 and c=:3 and a is not null and b is not null and c is not null"

drop table test_idx_tab purge
 drop table test_idx_tab succeeded.

create table test_idx_tab(a, b, c, str) as 
select nullif(mod(rownum,123),0), nullif(mod(rownum,5),0), nullif(mod(rownum,7),0), rpad(rownum,300,'x')
from dual connect by level < 1000
create table succeeded.

create index test_idx_tab$comp on test_idx_tab(a, b, c)
create index succeeded.

select a,b,c from test_idx_tab where a is null and b = 4 and c = 5

A                                       B                                       C                                       
--------------------------------------- --------------------------------------- --------------------------------------- 
                                        4                                       5                                       

1 rows selected

select * from table(dbms_xplan.display_cursor())

PLAN_TABLE_OUTPUT                                                                                                                                                                                                                                                                                            
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ 
SQL_ID  9x63nrma9uxs7, child number 1                                                                                                                                                                                                                                                                        
-------------------------------------                                                                                                                                                                                                                                                                        
select a,b,c from test_idx_tab where a is null and b = 4 and c = 5                                                                                                                                                                                                                                           
                                                                                                                                                                                                                                                                                                             
Plan hash value: 2398808346                                                                                                                                                                                                                                                                                  
                                                                                                                                                                                                                                                                                                             
--------------------------------------------------------------------------------------                                                                                                                                                                                                                       
| Id  | Operation        | Name              | Rows  | Bytes | Cost (%CPU)| Time     |                                                                                                                                                                                                                       
--------------------------------------------------------------------------------------                                                                                                                                                                                                                       
|   0 | SELECT STATEMENT |                   |       |       |     1 (100)|          |                                                                                                                                                                                                                       
|*  1 |  INDEX RANGE SCAN| TEST_IDX_TAB$COMP |     1 |    39 |     1   (0)| 00:00:01 |                                                                                                                                                                                                                       
--------------------------------------------------------------------------------------                                                                                                                                                                                                                       
                                                                                                                                                                                                                                                                                                             
Predicate Information (identified by operation id):                                                                                                                                                                                                                                                          
---------------------------------------------------                                                                                                                                                                                                                                                          
                                                                                                                                                                                                                                                                                                             
   1 - access("A" IS NULL AND "B"=4 AND "C"=5)                                                                                                                                                                                                                                                               
       filter(("B"=4 AND "C"=5))                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                             
Note                                                                                                                                                                                                                                                                                                         
-----                                                                                                                                                                                                                                                                                                        
   - dynamic sampling used for this statement (level=2)                                                                                                                                                                                                                                                      
                                                                                                                                                                                                                                                                                                             

23 rows selected