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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