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数据库索引实例之二consistent gets

数据来源

根据博客：某社区600万用户数据导入MYSQL、MSSQL、Oracle数据库方法，我们得到了一个含有600多万条用户数据的oracle数据库。本文就是根据这个来验证数据库索引的特性。

1.测试数据库CSDNUSER

View Code

CREATE TABLE "SCOTT"."CSDNUSER"
  (
    "ID" int primary key not null,
    "USERNAME"    VARCHAR2(256),
    "PASSWORD"   VARCHAR2(256),
    "EMAIL" VARCHAR2(256)
  )

数据导入方法参考某社区600万用户数据导入MYSQL、MSSQL、Oracle数据库方法。

上述数据库包含主键索引，但是没有为主键命名，因此搜索该索引的等级BLEVEL，可以通过以下查询语句求出：

select index_name, blevel, num_rows from user_indexes where table_name = 'CSDNUSER';

查询结果如下图所示：

从上述查询结果中我们发现没有BLEVEL和NUM_ROWS。

1.未命名的主键

接下来我们查询ID>700万数据，之所以是700万是因为我们总共数据时600多万，这样可以更加明显的看出来有没有索引的查询效率。查询语句如下：

SET AUTOTRACE ON
SELECT * FROM CSDNUSER2 WHERE ID>7000000;

查询执行计划如下：

View Code

执行计划
----------------------------------------------------------
Plan hash value: 2317779687

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

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

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

|   0 | SELECT STATEMENT            |              |     1 |   403 |     1   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| CSDNUSER2    |     1 |   403 |     1   (0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN          | SYS_C0038672 |     1 |       |     1   (0)| 00:00:01 |

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


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

   2 - access("ID">7000000)

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
          9  recursive calls
          0  db block gets
         92  consistent gets
          2  physical reads
        116  redo size
        450  bytes sent via SQL*Net to client
        405  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          0  rows processed

从统计信息中我们看出一共有“92 consistent gets”，相当于有92次IO。

2.无主键

然后我们删除上述主键SYS_COO38672，删除语句如下：

--删除主键
alter table csdnuser2 drop constraint SYS_C0038672;

再次执行上述查询语句，查询执行计划如下：

View Code

执行计划
----------------------------------------------------------
Plan hash value: 1618046436

-------------------------------------------------------------------------------
| Id  | Operation         | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |           |   498 |   195K| 12348   (1)| 00:02:29 |
|*  1 |  TABLE ACCESS FULL| CSDNUSER2 |   498 |   195K| 12348   (1)| 00:02:29 |
-------------------------------------------------------------------------------

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

   1 - filter("ID">7000000)

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
        182  recursive calls
          0  db block gets
      45129  consistent gets
      45015  physical reads
          0  redo size
        450  bytes sent via SQL*Net to client
        405  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          5  sorts (memory)
          0  sorts (disk)
          0  rows processed

从统计信息中我们看出一共有“45129 consistent gets”，相当于有45129次IO。

3.添加命名主键

添加主键语句如下：

--添加主键
alter table csdnuser add constraint pk_csdnuser primary key(ID);

查询该索引的等级BLEVEL，可以通过以下查询语句求出：

select index_name, blevel, num_rows from user_indexes where table_name = 'CSDNUSER';

查询结果如下图所示：

从上述查询结果中我们发现BLEVEL：2和NUM_ROWS=6428632，为表中记录数。

再次执行上述查询语句，查询执行计划如下：

View Code

执行计划
----------------------------------------------------------
Plan hash value: 2702699671

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

-----------

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

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

|   0 | SELECT STATEMENT            |             |     1 |   403 |     2   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| CSDNUSER    |     1 |   403 |     2   (0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN          | PK_CSDNUSER |     1 |       |     2   (0)| 00:00:01 |

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


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

   2 - access("ID">7000000)


统计信息
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
        450  bytes sent via SQL*Net to client
        405  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          0  rows processed

发现是“ 3 consistent gets”，表明添加命名索引以后，只需要3次IO就可以结束查询。

PS：2012-6-13解释前面错误理解

上述的命名主键与非命名主键的说法是错误的，第二次consistent gets子所以很大是因为删除了主键索引，这是没有错的。而第三次的consistent gets为3，而第一次consistent gets为92，并不说明自定义命名的索引效率比系统命名的索引效率高。之所以第三次只需要3次consistent gets是因为执行完第一次以后有缓存存在。假设在第一次查询以后再一次查询，那么统计结果跟第三次一模一样。

2.测试数据库USERINFO

http://www.itpub.net/thread-1313899-1-1.html

2.1创建数据库USERINFO

View Code

CREATE TABLE "USERINFO"
  (
    "NO" INT ,
    "NAME"    VARCHAR2(50),
    "BIRTHDAY"   DATE
  )

2.2创建序列

View Code

CREATE  SEQUENCE SEQ_USERINFO_NO
INCREMENT BY 1   -- 每次加几个  
START WITH 1     -- 从1开始计数

2.3插入100000条数据

View Code

begin
  for i in 1..100000 loop
      INSERT INTO USERINFO VALUES(SEQ_USERINFO_NO.nextval,'XUWEI',SYSDATE);
  end loop;
end;
/

2.4统计结果

查询NO=5000，查询语句如下：

View Code

set autotrace traceonly
select * from userinfo where no = 5000;

第一次查询得到的统计信息：

View Code

未选定行


执行计划
----------------------------------------------------------
Plan hash value: 3576123897

------------------------------------------------------------------------------
| Id  | Operation         | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |          |     1 |    49 |   102   (0)| 00:00:02 |
|*  1 |  TABLE ACCESS FULL| USERINFO |     1 |    49 |   102   (0)| 00:00:02 |
------------------------------------------------------------------------------

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

   1 - filter("NO"=5000)

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
          5  recursive calls
          0  db block gets
     119023  consistent gets
          0  physical reads
      25048  redo size
        391  bytes sent via SQL*Net to client
        405  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          0  rows processed

第二次查询得到的统计信息：

View Code

未选定行


执行计划
----------------------------------------------------------
Plan hash value: 3576123897

------------------------------------------------------------------------------
| Id  | Operation         | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |          |     1 |    49 |   102   (0)| 00:00:02 |
|*  1 |  TABLE ACCESS FULL| USERINFO |     1 |    49 |   102   (0)| 00:00:02 |
------------------------------------------------------------------------------

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

   1 - filter("NO"=5000)

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
     100704  consistent gets
          0  physical reads
      21164  redo size
        391  bytes sent via SQL*Net to client
        405  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          0  rows processed

第三次查询得到的统计信息：

View Code

未选定行


执行计划
----------------------------------------------------------
Plan hash value: 3576123897

------------------------------------------------------------------------------
| Id  | Operation         | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |          |     1 |    49 |   102   (0)| 00:00:02 |
|*  1 |  TABLE ACCESS FULL| USERINFO |     1 |    49 |   102   (0)| 00:00:02 |
------------------------------------------------------------------------------

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

   1 - filter("NO"=5000)

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
     100704  consistent gets
          0  physical reads
      21208  redo size
        391  bytes sent via SQL*Net to client
        405  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          0  rows processed

为NO字段添加索引IX_USERINFO_NO

View Code

--创建索引
create index IX_USERINFO_NO on USERINFO(NO);

第四次查询得到的统计信息：

View Code

执行计划
----------------------------------------------------------
Plan hash value: 1066629497

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

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

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

|   0 | SELECT STATEMENT            |                |     1 |    49 |     2   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| USERINFO       |     1 |    49 |     2   (0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN          | IX_USERINFO_NO |     1 |       |     1   (0)| 00:00:01 |

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


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

   2 - access("NO"=5000)

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
          9  recursive calls
          0  db block gets
         76  consistent gets
          1  physical reads
          0  redo size
        543  bytes sent via SQL*Net to client
        416  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

第五次查询得到的统计信息：

View Code

执行计划
----------------------------------------------------------
Plan hash value: 1066629497

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

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

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

|   0 | SELECT STATEMENT            |                |     1 |    49 |     2   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| USERINFO       |     1 |    49 |     2   (0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN          | IX_USERINFO_NO |     1 |       |     1   (0)| 00:00:01 |

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


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

   2 - access("NO"=5000)

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          4  consistent gets
          0  physical reads
          0  redo size
        543  bytes sent via SQL*Net to client
        416  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

删除索引IX_USERINFO_NO

View Code

--删除索引
drop index IX_USERINFO_NO;

添加主键PK_USERINFO_NO

View Code

--添加主键
alter table USERINFO add constraint PK_USERINFO_NO primary key(NO);

第六次查询得到的统计信息：

View Code

执行计划
----------------------------------------------------------
Plan hash value: 4161181038

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

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

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

|   0 | SELECT STATEMENT            |                |     1 |    49 |     2   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| USERINFO       |     1 |    49 |     2   (0)| 00:00:01 |

|*  2 |   INDEX UNIQUE SCAN         | PK_USERINFO_NO |     1 |       |     1   (0)| 00:00:01 |

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


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

   2 - access("NO"=5000)


统计信息
----------------------------------------------------------
        192  recursive calls
          0  db block gets
         33  consistent gets
          1  physical reads
          0  redo size
        447  bytes sent via SQL*Net to client
        405  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          6  sorts (memory)
          0  sorts (disk)
          1  rows processed

第七次查询得到的统计信息：

View Code

执行计划
----------------------------------------------------------
Plan hash value: 4161181038

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

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

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

|   0 | SELECT STATEMENT            |                |     1 |    49 |     2   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| USERINFO       |     1 |    49 |     2   (0)| 00:00:01 |

|*  2 |   INDEX UNIQUE SCAN         | PK_USERINFO_NO |     1 |       |     1   (0)| 00:00:01 |

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


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

   2 - access("NO"=5000)


统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
        447  bytes sent via SQL*Net to client
        405  bytes received via SQL*Net from client
          1  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

2.5统计分析

从第一次到第三次查询，都是无索引状态下的查询，我们可以发现：

第一次查询时recursive calls=5>0，而后面两次recursive calls都为0，这个也适用于后期有索引的情况
consistent gets在不断缩小，知道最后不变。例如第一次查询的consistent gets最大，而第二次和第三次的consistent gets相等。
在三次查询过程中，physical reads都为0。（physical reads=0表明物理IO次数为0，也就是说没有从硬盘上读取数据到内存中。之所以physical reads=0，是因为前面insert的100000数据已经在buffer_cache里了）

从第四次到第五次查询，都是有索引状态下的插叙，我们可以发现：

consistent gets次数在下降，consistent gets最后变到4。
recursive calls次数在下降，recursive calls最后变到0。
相对于第一次到第三次查询，consistent gets明显下降，这是因为添加了索引，前面第一次到第三次是全表扫描，而第四次跟第五次查询是索引扫描。逻辑读（consistent gets）之所以最后会是4，是因此需要读取根节点一个，分支一个，叶子一个，表块一个，共4个。
physical reads同上。

从六次到第七次查询，是将原来索引换成了主键，我们可以发现：

consistent gets最后降为3，而不是4，这个是不明白的地方。
consistent gets同上
recursive calls同上
physical reads同上

主键也是索引的一种，只要加了索引，那么逻辑读（consistent gets）就明显降低，查询效率大大提高。这也是索引的作用。

2.6清空缓存后的physical reads

假设我们运行如下命令清空缓存：

alter system flush buffer_cache;
alter system flush shared_pool;

然后再次执行查询语句，得到的统计信息如下：

View Code

执行计划
----------------------------------------------------------
Plan hash value: 1066629497

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

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

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

|   0 | SELECT STATEMENT            |                |     1 |    49 |     2   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| USERINFO       |     1 |    49 |     2   (0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN          | IX_USERINFO_NO |     1 |       |     1   (0)| 00:00:01 |

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


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

   2 - access("NO"=5000)

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
        401  recursive calls
          0  db block gets
        128  consistent gets
        308  physical reads
          0  redo size
        543  bytes sent via SQL*Net to client
        416  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          6  sorts (memory)
          0  sorts (disk)
          1  rows processed

可以看到physical reads=308。

再次执行查询语句，，得到的统计信息如下：

View Code

执行计划
----------------------------------------------------------
Plan hash value: 1066629497

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

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

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

|   0 | SELECT STATEMENT            |                |     1 |    49 |     2   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| USERINFO       |     1 |    49 |     2   (0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN          | IX_USERINFO_NO |     1 |       |     1   (0)| 00:00:01 |

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


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

   2 - access("NO"=5000)

Note
-----
   - dynamic sampling used for this statement


统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          4  consistent gets
          0  physical reads
          0  redo size
        543  bytes sent via SQL*Net to client
        416  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

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原文地址：https://www.cnblogs.com/xwdreamer/p/2545689.html