count(*) and count(1)

gistfile1.sql

SQL> create table t_nulls as select cast(null as int) col_nulls from dual connect by level<=1e3;

Table created.

SQL> select count(*) from t_nulls;

  COUNT(*)
----------
      1000

1 row selected.

SQL> select count(1) from t_nulls;

  COUNT(1)
----------
      1000

1 row selected.

SQL> explain plan for select count(*) from t_nulls;

Explained.

SQL> @xplan

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------
Plan hash value: 2660291840

----------------------------------------------------------------------
| Id  | Operation          | Name    | Rows  | Cost (%CPU)| Time     |
----------------------------------------------------------------------
|   0 | SELECT STATEMENT   |         |     1 |     3   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE    |         |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T_NULLS |  1000 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------

Column Projection Information (identified by operation id):
-----------------------------------------------------------

   1 - (#keys=0) COUNT(*)[22]

SQL> explain plan for select count(1) from t_nulls;

Explained.

SQL> @xplan

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------
Plan hash value: 2660291840

----------------------------------------------------------------------
| Id  | Operation          | Name    | Rows  | Cost (%CPU)| Time     |
----------------------------------------------------------------------
|   0 | SELECT STATEMENT   |         |     1 |     3   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE    |         |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T_NULLS |  1000 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------

Column Projection Information (identified by operation id):
-----------------------------------------------------------

   1 - (#keys=0) COUNT(*)[22]