Table Access by Rowid 有时候会花费很长时间

通过ROWID访问表的时候, 通常是先根据索引找到某个范围的ROWID,然后再根据ROWID反查表去获得满足其他条件的数据。　通常反查的物理读会比较多。　如果这个反查的物理读特别大并且没有其他更有效的方式过滤返回结果的时候，应该考虑检查使用的索引是否不合适

官方解释如下：

8.2.3.2 How Table Access by Rowid Works

To access a table by rowid, the database performs multiple steps.

The database does the following:

Obtains the rowids of the selected rows, either from the statement WHERE clause or through an index scan of one or more indexes
Table access may be needed for columns in the statement not present in the index.
Locates each selected row in the table based on its rowid

8.2.3.3 Table Access by Rowid: Example

This example demonstrates rowid access of the hr.employees table.

Assume that you run the following query:

SELECT * 
FROM   employees 
WHERE  employee_id > 190;

Step 2 of the following plan shows a range scan of the emp_emp_id_pk index on the hr.employees table. The database uses the rowids obtained from the index to find the corresponding rows from the employees table, and then retrieve them. The BATCHED access shown in Step 1 means that the database retrieves a few rowids from the index, and then attempts to access rows in block order to improve the clustering and reduce the number of times that the database must access a block.

--------------------------------------------------------------------------------
|Id| Operation                           | Name     |Rows|Bytes|Cost(%CPU)|Time|
--------------------------------------------------------------------------------
| 0| SELECT STATEMENT                    |             |  |    |2(100)|        |
| 1|  TABLE ACCESS BY INDEX ROWID BATCHED|EMPLOYEES    |16|1104|2  (0)|00:00:01|
|*2|   INDEX RANGE SCAN                  |EMP_EMP_ID_PK|16|    |1  (0)|00:00:01|
--------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   2 - access("EMPLOYEE_ID">190)