Index is not executing

Question

Hi All.
I am executing a SQL on a table, I have three different indexes on three different column let say, table have a, b, c, d, e, f, g, h, i , j.
Index created on c, d, g
Column "C" and "D" have values in every row, while "G" dont have value in any row.
Index not executed when I wrote following where clause:
Case 1
Where C is not null
Case 2
Where D is not null
While index executed when I wrote where clause as
where "G" is not null;
Whats wrong with it?
Hassan

Answer 1

In some cases Oracle can also choose Fast Full Scan in case "C" and "D". Please look at the example below:
[email protected]> select index_name from user_indexes where table_name = 'PRODUCTS'
2 /
no rows selected
[email protected]> create index products_name_idx on products(prod_name) compute statistics;
Index created.
[email protected]> set autotrace traceonly explain statistics
[email protected]> select prod_name from products where prod_name is not null;
10000 rows selected.
Execution Plan
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=14 Card=10000 Bytes=
250000)
1 0 INDEX (FAST FULL SCAN) OF 'PRODUCTS_NAME_IDX' (NON-UNIQUE)
(Cost=14 Card=10000 Bytes=250000)
Statistics
117 recursive calls
0 db block gets
775 consistent gets
51 physical reads
0 redo size
150267 bytes sent via SQL*Net to client
7825 bytes received via SQL*Net from client
668 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10000 rows processed
Here there is no need to read table data because Oracle can satisfy this query using just data that is in the index blocks
Best Regards
Krystian Zieja / mob