Two cols index used when "col1=:1" but not when "col1 is null"(same cardin)

Question

Hi everybody,
here is my test case (Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - 64bi):
DROP TABLE TEST;
CREATE TABLE TEST AS SELECT RPAD('X',100,'X') X, (CASE WHEN MOD(ROWNUM,10)=0 THEN CAST(NULL AS NUMBER) ELSE MOD(ROWNUM,10) END) Y, ROWNUM Z FROM DUAL CONNECT BY LEVEL<=10000;
SELECT Y, COUNT(*) FROM TEST GROUP BY Y ORDER BY 1;
create index test_idx on test(y,z);
exec dbms_stats.gather_table_stats(ownname=>'&_USER', tabname=>'TEST', method_opt=>'for all columns size REPEAT', cascade=>true, no_invalidate=>false);
select * from (select z from test where y=1 order by z) where rownum <= 10;
select * from (select z from test where y is null order by z) where rownum <= 10;So there are 1000 records per Y column value:
SQL> SELECT Y, COUNT(*) FROM TEST GROUP BY Y ORDER BY 1;
         Y   COUNT(*)
         1       1000
         2       1000
         3       1000
         4       1000
         5       1000
         6       1000
         7       1000
         8       1000
         9       1000
                 1000
10 filas seleccionadas.So, what I don't undertand is why in the second query from below, the index is not used:
  select * from (select z from test where y=1 order by z) where rownum <= 10
Plan hash value: 2425049421
| Id  | Operation          | Name     | Starts | E-Rows | Cost (%CPU)| A-Rows |   A-Time   | Buffers |
|*  1 |  COUNT STOPKEY     |          |      1 |        |            |     10 |00:00:00.01 |       3 |
|   2 |   VIEW             |          |      1 |     10 |     2   (0)|     10 |00:00:00.01 |       3 |
|*  3 |    INDEX RANGE SCAN| TEST_IDX |      1 |     10 |     2   (0)|     10 |00:00:00.01 |       3 |
Predicate Information (identified by operation id):
   1 - filter(ROWNUM<=10)
   3 - access("Y"=1)
  select * from (select z from test where y is null order by z) where rownum <= 10
Plan hash value: 2549875903
| Id  | Operation               | Name | Starts | E-Rows | Cost (%CPU)| A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
|*  1 |  COUNT STOPKEY          |      |      1 |        |            |     10 |00:00:00.01 |     161 |       |       |          |
|   2 |   VIEW                  |      |      1 |   1000 |    43   (5)|     10 |00:00:00.01 |     161 |       |       |          |
|*  3 |    SORT ORDER BY STOPKEY|      |      1 |   1000 |    43   (5)|     10 |00:00:00.01 |     161 |  9216 |  9216 | 8192  (0)|
|*  4 |     TABLE ACCESS FULL   | TEST |      1 |   1000 |    42   (3)|   1000 |00:00:00.01 |     161 |       |       |          |
Predicate Information (identified by operation id):
   1 - filter(ROWNUM<=10)
   3 - filter(ROWNUM<=10)
   4 - filter("Y" IS NULL)Any clue?
Thanks.
Joaquin Gonzalez

Answer 1

Hi,
Null values are not stored in the index, so if you use "col is null" as a predicate there is no entry on the index and so no pointer to the actual row hence the full table scan.
If you query USER_INDEXES and look in the DISTINCT_KEYS column for your index, you should see that there are only 9 keys.
e.g.
SQL> create table nt as
  2  select decode(m, 0, null, m) v from
  3* (select mod(level,10) m from dual connect by level <= 10000)
SQL> /
Table created.
SQL> select v, count(*) from nt group by v order by v ;
V                                          COUNT(*)
1                                              1000
2                                              1000
3                                              1000
4                                              1000
5                                              1000
6                                              1000
7                                              1000
8                                              1000
9                                              1000
*NULL*                                         1000
10 rows selected.
SQL> create index nt_i on nt (v) ;
Index created.
SQL> analyze table nt compute statistics
  2  /
SQL> select distinct_keys from user_indexes where index_name = 'NT_I'
SQL> /
DISTINCT_KEYS
            9Edited by: Steve_B on 06-Jul-2009 02:30