explain plan for
selec ........；

select * from table(dbms_xplan.display)
test了一下：

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

Explained

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
-----------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost |
-----------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 2 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FAST FULL SCAN| PKY_EMPLOYEE | 4409 | | 2 |
-----------------------------------------------------------------------
Note: cpu costing is off, PLAN_TABLE' is old version

10 rows selected

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

Explained

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost |
------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | |
| 1 | SORT AGGREGATE | | | | |
| 2 | TABLE ACCESS FULL | INV_TRANSACTIONS_LOG | | | |
------------------------------------------------------------------------------
Note: rule based optimization, PLAN_TABLE' is old version

10 rows selected