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

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

 

ORACLEdatabasedbaselect
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.