you don't need to.  An index on f_seq, t_seq would accomplish that.  

It is that ENTIRELY null entries are not made into a b*tree.  If you have a concatenated index (on more then one column) and AT LEAST one of the columns is not null -- that'll find it's way into the index.  

Consider:

ops$tkyte@ORA920> create table t ( f_seq int, t_seq int, x char(1) );

Table created.

ops$tkyte@ORA920>
ops$tkyte@ORA920> create index t_idx on t(f_seq,t_seq);

Index created.

ops$tkyte@ORA920>
ops$tkyte@ORA920> set autotrace on explain
ops$tkyte@ORA920> select f_seq, t_seq
  2    from t
  3   where f_seq > 0 and t_seq is null;

no rows selected


Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE
   1    0   INDEX (RANGE SCAN) OF 'T_IDX' (NON-UNIQUE)



ops$tkyte@ORA920>
ops$tkyte@ORA920> select f_seq, t_seq, x
  2    from t
  3   where f_seq > 0 and t_seq is null;

no rows selected


Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE
   1    0   TABLE ACCESS (BY INDEX ROWID) OF 'T'
   2    1     INDEX (RANGE SCAN) OF 'T_IDX' (NON-UNIQUE)



ops$tkyte@ORA920> set autotrace off

<b>see how the first query doesn't access the table -- the index is sufficient to answer the question -- hence it is getting f_seq and t_seq from the index -- the null entry is indexed in this case (since f_seq is not null)
</b>

 

any time you have a concatenated index (index with more then one key in it) AND one of the columns in the index is NOT NULL (the constant "1" is not null in your example) -- then an index entry is made.

The thing is -- an index on an entirely NULL KEY is not made into a b*tree.  You just have an example where an entry is always made in the b*tree..

it works -- it is just "bigger".  Also -- it is a FUNCTION BASED INDEX so it'll not work in 8.0 and before:

ops$tkyte@ORA806> create index t_idx on t(x,1);
create index t_idx on t(x,1)
                          *
ERROR at line 1:
ORA-00904: invalid column name

and it requires the CBO (that index is invisible to the RBO) as is the bitmap though ;) 

RR is NOT determistic either!!


ops$tkyte@ORA9IR2> create table t ( x date );

Table created.

ops$tkyte@ORA9IR2> create index t_idx on t( case when x > to_date('01-jan-05','dd-mon-rr') then 1 end );
create index t_idx on t( case when x > to_date('01-jan-05','dd-mon-rr') then 1 end )
                                               *
ERROR at line 1:
ORA-01743: only pure functions can be indexed


ops$tkyte@ORA9IR2> create index t_idx on t( case when x > to_date('01-jan-2005','dd-mon-yyyy') then 1 end );

Index created.

 

ops$tkyte%ORA10GR2> create table t as select * from all_objects;

Table created.

ops$tkyte%ORA10GR2> exec dbms_stats.gather_table_stats( user, 'T' );

PL/SQL procedure successfully completed.

ops$tkyte%ORA10GR2> set autotrace traceonly explain
ops$tkyte%ORA10GR2> select * from t where status is null;

Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |    93 |   155   (2)| 00:00:02 |
|*  1 |  TABLE ACCESS FULL| T    |     1 |    93 |   155   (2)| 00:00:02 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("STATUS" IS NULL)

ops$tkyte%ORA10GR2> create index t_idx on t(status,0);

Index created.

ops$tkyte%ORA10GR2> select * from t where status is null;

Execution Plan
----------------------------------------------------------
Plan hash value: 470836197

-------------------------------------------------------------------------------------
| Id  | Operation                   | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |       |     1 |    93 |     1   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T     |     1 |    93 |     1   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | T_IDX |     1 |       |     1   (0)| 00:00:01 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("STATUS" IS NULL)

ops$tkyte%ORA10GR2> set autotrace off