select count(column_name) form table_name or select count(*) from table_Name

select count(*) from table_name

select count(column_name) form table_name

To calculate a table's row number, most programer think 'select count(*) from table_name'   is not a patch on ' select count(column_name) form table_name'. Is it ?

example:

select count(*) from emp;

Execution Plan

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

Plan hash value: 2937609675

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

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

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

|   0 | SELECT STATEMENT |        |     1 |     1   (0)| 00:00:01 |

|   1 |  SORT AGGREGATE  |        |     1 |            |          |

|   2 |   INDEX FULL SCAN| PK_EMP |    14 |     1   (0)| 00:00:01 |

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

select count(ename) from emp;

Execution Plan

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

Plan hash value: 2083865914

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

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

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

|   0 | SELECT STATEMENT   |      |     1 |     6 |     3   (0)| 00:00:01 |

|   1 |  SORT AGGREGATE    |      |     1 |     6 |            |          |

|   2 |   TABLE ACCESS FULL| EMP  |    14 |    84 |     3   (0)| 00:00:01 |

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

It is obviously see that the first sql need less cost of the oracle resource.

When we do select count(*) from table_name, oracle cbo will do a internal tuning which will select the most performance path to do that.