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.
 
 

补充：综合编程 , 其他综合 ,

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