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HomeDatabaseMysql Tutorial 用示例说明索引数据块中出现热块&Latch的场景,并给出解决方案

引言:索引的热块其实和数据块的热块发生的原理大相径庭,也都是因为大量会话一起访问同一个索引块造成的,我们的解决方案有反向索引,分区索引等。我们说任何一

引言:索引的热块其实和数据块的热块发生的原理大相径庭,也都是因为大量会话一起访问同一个索引块造成的,我们的解决方案有反向索引,分区索引等。我们说任何一种方式都不是完美的,有优点就必然有缺点,我们把包含索引键值的索引块从顺序排列打散到无序排列,香港空间,降低了latch争用,同时也增加了oracle扫描块的数量。我们在实际使用时多测试取长补短,以提高系统的整体性能为目标。


 

LEO1@LEO1>create table leo1 (id  number , name  varchar2(200));     创建了一个leo1表

Table created.

LEO1@LEO1>insert into leo1 (id,name) select object_id,object_name from dba_objects; 将dba_objects前2个字段复制到leo1表中。

71966 rowscreated.

LEO1@LEO1>select id,name from leo1 where rownum

        ID NAME

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

       673 CDC_CHANGE_SOURCES$

       674 I_CDC_CHANGE_SOURCES$

       675 CDC_CHANGE_SETS$

       676 I_CDC_CHANGE_SETS$

       677 CDC_CHANGE_TABLES$

       678 I_CDC_CHANGE_TABLES$

       679 CDC_SUBSCRIBERS$

       680 I_CDC_SUBSCRIBERS$

       681 CDC_SUBSCRIBED_TABLES$

LEO1@LEO1>create index leo1_index on leo1(id);     在leo1表上id列创建一个索引

Index created.

LEO1@LEO1>execute dbms_stats.gather_table_stats('LEO1','LEO1',cascade=>true);  对表和索引一起做一个分析,香港空间,cascade=>true 指的是级联表上的索引一起做分析

PL/SQL proceduresuccessfully completed.

LEO1@LEO1>create table leo2 (id number,name varchar2(200));      创建leo2表

Table created.

LEO1@LEO1>insert into leo2 (id,name) select object_id,object_name from dba_objects;  插入71968行

71968 rowscreated.

为什么比leo1表多了2行呢,就是多了leo1和leo1_index这2个对象,我们刚刚建的。

LEO1@LEO1>create index leo2_index on leo2(id) reverse;        创建一个反向索引

Index created.

LEO1@LEO1>execute dbms_stats.gather_table_stats('LEO1','LEO2',cascade=>true);  做分析

PL/SQL proceduresuccessfully completed.

LEO1@LEO1>select index_name,index_type,table_name,status from dba_indexes wheretable_name in ('LEO1','LEO2');

INDEX_NAME   INDEX_TYPE      TABLE_NAME      STATUS

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

LEO1_INDEX    NORMAL                LEO1           VALID

LEO2_INDEX    NORMAL/REV       LEO2           VALID  

LEO2_INDEX   是反向索引,我们使用它来把顺序的索引块反向成无序索引块存储,这样我们在查询一个区间范围时,索引键值就会落在不连续的索引块上,防止热块的产生,降低“latch 链表”争用。这可能算是反向索引唯一被使用的情况。因为反向索引不支持index range scan功能,只支持index full scan 全索引扫描,如何理解呢,虚拟主机,举个简单的例子 反向索引 不能帮你检索出  id> 1 and id

LEO1@LEO1> set   autotrace  on;       启动执行计划

LEO1@LEO1>select count(*)  from leo1 whereid

  COUNT(*)

----------

        98

Execution Plan

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

Plan hash value:423232053

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

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

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

|   0 | SELECT STATEMENT  |           |     1 |     5 |    2   (0)| 00:00:01 |

|   1 | SORT AGGREGATE   |           |     1|     5 |            |          |

|*  2 |   INDEX RANGE SCAN| LEO1_INDEX |    96 |  480 |     2   (0)| 00:00:01 |

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

索引范围扫描,因为我们查询索引键值都是存放在连续的索引块中,所以只有仅仅的2个一致性读,它只扫描符合条件的索引块就能找到相应的记录。

PredicateInformation (identified by operation id):

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

   2 - access("ID"

Statistics

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

          0 recursive calls

          0 db block gets

          2  consistent gets

          0 physical reads

          0 redo size

        526 bytes sent via SQL*Net to client

        523 bytes received via SQL*Net from client

          2 SQL*Net roundtrips to/from client

          0 sorts (memory)

          0 sorts (disk)

          1 rows processed

LEO1@LEO1>select count(*)  from leo2 whereid

  COUNT(*)

----------

        98

Execution Plan

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

Plan hash value:1710468575

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

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

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

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