数据库的作用就是实现对数据的管理和查询。任何一个数据库系统，必然存在对数据的大量读或者写或者两中操作都大量存在。IO问题也往往是导致数据库性能问题的重要原因。在这篇文章中，主要帮助大家在理解Oracle的读写操作机制的基础上，灵活解决遇到的各种常见的IO问题。

• 当服务进程扫描一定数量（阀值）的Buffer Cache后还没有找到干净、可重用的缓存块后，它会通知DBWn进程将“脏”数据写入文件中去，以释放出空闲缓存；
• 当发生检查点（Checkpoint）时。

• 当一个事务提交（COMMIT）时；
• 每3秒钟写一次Log Buffer；
• 当Log Buffer到达1/3满时；
• 当DBWn进程将“脏”数据写入磁盘时；

• 临时表/索引

• 排序

SQL> alter session set sort_area_size = 10000000;

Session altered.

SQL> select owner, object_name from t_test1

  2  order by object_id;

47582 rows selected.

Execution Plan

----------------------------------------------------------

Plan hash value: 1312425564

------------------------------------------------------------------------------

| Id  | Operation          | Name    | Rows  | Bytes | Cost (%CPU)| Time     |

------------------------------------------------------------------------------

|   0 | SELECT STATEMENT   |         | 47582 |  1486K|   155   (4)| 00:00:02 |

|   1 |  SORT ORDER BY     |         | 47582 |  1486K|   155   (4)| 00:00:02 |

|   2 |   TABLE ACCESS FULL| T_TEST1 | 47582 |  1486K|   150   (1)| 00:00:02 |

------------------------------------------------------------------------------

Statistics

----------------------------------------------------------

          1  recursive calls

          0  db block gets

        658  consistent gets

          0  physical reads

          0  redo size

    1566184  bytes sent via SQL*Net to client

      35277  bytes received via SQL*Net from client

       3174  SQL*Net roundtrips to/from client

          1  sorts (memory)

          0  sorts (disk)

      47582  rows processed

SQL> alter session set sort_area_size = 10000;

Session altered.

SQL> select owner, object_name from t_test1

  2  order by object_id;

47582 rows selected.

Execution Plan

----------------------------------------------------------

Plan hash value: 1312425564

--------------------------------------------------------------------------------

| Id  | Operation          | Name    | Rows  | Bytes |TempSpc| Cost (%CPU)| Time|

--------------------------------------------------------------------------------

|   0 | SELECT STATEMENT   |         | 47582 |  1486K|       |  1251   (1)| 00:0

0:16 |

|   1 |  SORT ORDER BY     |         | 47582 |  1486K|  4136K|  1251   (1)| 00:0

0:16 |

|   2 |   TABLE ACCESS FULL| T_TEST1 | 47582 |  1486K|       |   150   (1)| 00:0

0:02 |

---------------------------------------------------------------------------------

Statistics

----------------------------------------------------------

          6  recursive calls

         20  db block gets

        658  consistent gets

        629  physical reads

          0  redo size

    1566184  bytes sent via SQL*Net to client

      35277  bytes received via SQL*Net from client

       3174  SQL*Net roundtrips to/from client

          0  sorts (memory)

          1  sorts (disk)

      47582  rows processed 

• 临时LOB对象