Why is my SQL not using an index?

Question

I have a small SQL query (10g) where I join to basic table together on a customer_id column.
select *
from customer c
inner join work_item sp1 ON sp1.customer_id = c.customer_id
and I am using TOAD, which tells me (in the Explain Plan area), that I'm doing a "TABLE ACCESS FULL" scan on the customer table.
But when I look at the details of BOTH tables, I can clearly see that they both have an index on the customer field, and they are both the same data types...
The interesting thing, is that if I change the query to only select data from the work_item table,
select sp1.*
from customer c
inner join work_item sp1 ON sp1.customer_id = c.customer_id
I can see that the indexes are being used...
Why is this? I must be able to select certain columns from the customer table, but why would it not use an index when I do?
Thanks!

Answer 1

If I understand correctly, below is a simple test case.
I created a small table and indexed "object_id" column.
When I issue "Select * from t" optimizer goes with FULL TABLE SCAN, whereas wehn "select OBJECT_ID from t" is issued, optimizer chooses to read from the index instead of accessing the table. This is because FAST FULL SCAN is far cheaper than a FTS in this case.
SQL> create table t as select object_id, object_name from all_objects;
Table created.
SQL> exec dbms_stats.gather_table_stats(user, 't');
PL/SQL procedure successfully completed.
SQL> create index t_idx1 on t(object_id);
Index created.
SQL> set autotrace traceonly exp
SQL> select * from t;
Execution Plan
Plan hash value: 1601196873
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT  |      | 37932 |  1074K|    55   (2)| 00:00:01 |
| 1 | TABLE ACCESS FULL| T | 37932 | 1074K| 55 (2)| 00:00:01 |
SQL> select object_id from t;
Execution Plan
Plan hash value: 3958994525
| Id  | Operation            | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT     |        | 37932 |   185K|    25   (0)| 00:00:01 |
| 1 | INDEX FAST FULL SCAN| T_IDX1 | 37932 | 185K| 25 (0)| 00:00:01 |
SQL>