Oracle SQL优化之buffer read

我们在优化Oracle SQL时，CBO方面COST固然重要，但有时候也须参考buffer read（consistent gets或者db block gets），尤其是在面对复杂的执行计划时，这个值直接决定着SQL执行效率的高低。参见下面例子：
选择的mc$ma_action_result表在status字段选择性比较低。

引用

SQL> select count(*) from MC$MA_ACTION_RESULT;

  COUNT(*)
----------
    240029

SQL> select count(distinct status) from MC$MA_ACTION_RESULT;

COUNT(DISTINCTSTATUS)
---------------------
                    7

当status字段有索引时，执行计划为如下，可以看到consistent gets为895

引用

SQL> set autot traceonly exp stat
SQL> select id,target_id,target_type,source_file_path from mc$ma_action_result
  2  where status='DELETED' order by updated_at asc;

10707 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3391357726

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

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

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

|   0 | SELECT STATEMENT             |                         | 10095 |   798K|
       |   673   (1)| 00:00:09 |

|   1 |  SORT ORDER BY               |                         | 10095 |   798K|
  2008K|   673   (1)| 00:00:09 |

|   2 |   TABLE ACCESS BY INDEX ROWID| MC$MA_ACTION_RESULT     | 10095 |   798K|
       |   479   (1)| 00:00:06 |

|*  3 |    INDEX RANGE SCAN          | MC$MA_ACTION_RESULT_IDX | 10251 |       |
       |    29   (0)| 00:00:01 |

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


Predicate Information (identified by operation id):
---------------------------------------------------

   3 - access("STATUS"='DELETED')


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
        895  consistent gets
          0  physical reads
          0  redo size
     732057  bytes sent via SQL*Net to client
       8335  bytes received via SQL*Net from client
        715  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
      10707  rows processed

当字段status无索引时，其consistent gets为5923

引用

SQL> drop index mc$ma_action_result_idx;

Index dropped.

SQL> alter system flush buffer_cache;

System altered.

SQL> select id,target_id,target_type,source_file_path from mc$ma_action_result
  2  where status='DELETED' order by updated_at asc;

10707 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 276506348

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

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

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

|   0 | SELECT STATEMENT   |                     | 10095 |   798K|       |  1478
   (2)| 00:00:18 |

|   1 |  SORT ORDER BY     |                     | 10095 |   798K|  2008K|  1478
   (2)| 00:00:18 |

|*  2 |   TABLE ACCESS FULL| MC$MA_ACTION_RESULT | 10095 |   798K|       |  1284
   (2)| 00:00:16 |

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


Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("STATUS"='DELETED')


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       5923  consistent gets
          0  physical reads
          0  redo size
     732047  bytes sent via SQL*Net to client
       8335  bytes received via SQL*Net from client
        715  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
      10707  rows processed）

从以上执行计划，我们可以判断如下：
在表格MC$MA_ACTION_RESULT DML操作比较少的前提下，在status字段上建立索引，SQL执行效率相对较高