Do I need an index

Hi,

Oracle database 11.2 on Linux

I have a large table T1 with 10 columns, col1, col2, col3. ........col10

T1 primary key is (col1,col2,col3,col4). which has an index on (col1,col2,col3,col4).

Do I need another index on col3 for statements like.

select col5 from T1 where col3='xx';

The explain plan below shows I need an index on col3

1- Without col3 index

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Plan hash value: 1119245414

----------------------------------------------------------------------------------------------

| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |

----------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | |     1 |   145 | 10770   (1)| 00:02:10 |

|* 1 |  TABLE ACCESS FULL| T1 |     1 |   145 | 10770   (1)| 00:02:10 |

----------------------------------------------------------------------------------------------

2- After adding the index

--------------------

Plan hash value: 3956034033

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| Id  | Operation | Name | Rows  | Bytes | Cost (%CPU)| Time     |