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OracleTABLEACCESSBYINDEXROWID说明

时间:2021-07-01 10:21:17 帮助过:68人阅读

一. 测试环境 SQL select * from v$version where rownum=1; BANNER -------------------------------------------------------------------------------- Oracle Database 11g Enterprise Edition Release11.2.0.3.0 - 64bit Production SQL create table d

一. 测试环境

SQL> select * from v$version where rownum=1;

BANNER

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

Oracle Database 11g Enterprise Edition Release11.2.0.3.0 - 64bit Production

SQL> create table dave as selectobject_id,object_name,object_type,created,timestamp,status from all_objects;

表已创建。

SQL> create table dave2 as select * from dave;

表已创建。

--收集统计信息,这里没有收集直方图:

SQL> exec dbms_stats.gather_table_stats(ownname=>'SYS',tabname =>'DAVE',estimate_percent => 10 ,method_opt =>'FORCOLUMNS size 1',degree=>10,cascade => true);

PL/SQL 过程已成功完成。

SQL> exec dbms_stats.gather_table_stats(ownname=>'SYS',tabname =>'DAVE2',estimate_percent => 10 ,method_opt =>'FORCOLUMNS size 1',degree=>10,cascade => true);

PL/SQL 过程已成功完成。

--避免其他影响,先刷新buffer cache

SQL> alter system flush buffer_cache;

系统已更改。

--查看全表扫描时的执行计划:

SQL> set autot traceonly

SQL> select d1.object_name,d2.object_type fromdave d1,dave2 d2 where d1.object_id=d2.object_id;

已选择72762行。

执行计划

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

Plan hash value: 3613449503

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

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

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

| 0 |SELECT STATEMENT | | 72520 | 3824K| | 695 (1)| 00:00:09 |

|* 1 | HASH JOIN | | 72520 | 3824K| 2536K| 695 (1)| 00:00:09 |

| 2 | TABLE ACCESS FULL| DAVE2 | 71990 | 1687K| | 213 (1)| 00:00:03 |

| 3 | TABLE ACCESS FULL| DAVE | 72520 | 2124K| | 213 (1)| 00:00:03 |

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

Predicate Information (identified by operation id):

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

1 -access("D1"."OBJECT_ID"="D2"."OBJECT_ID")

统计信息

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

0 recursive calls

0 db block gets

6353 consistent gets

1558 physical reads

0 redo size

3388939 bytes sent via SQL*Net toclient

53874 bytes received via SQL*Netfrom client

4852 SQL*Net roundtrips to/fromclient

0 sorts (memory)

0 sorts (disk)

72762 rows processed

--这里产生了1558的物理读

SQL>

--object_id上创建索引:

SQL> create index idx_dave_object_idon dave(object_id);

索引已创建。

SQL> create index idx_dave_object_id2 ondave2(object_id);

索引已创建。

--在次查看执行计划:

SQL> select d1.object_name,d2.object_type fromdave d1,dave2 d2 where d1.object_id=d2.object_id;

已选择72762行。

执行计划

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

Plan hash value: 3613449503

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

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

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

| 0 |SELECT STATEMENT | | 72520 | 3824K| | 695 (1)| 00:00:09 |

|* 1 | HASH JOIN | | 72520 | 3824K| 2536K| 695 (1)| 00:00:09 |

| 2 | TABLE ACCESS FULL| DAVE2 | 71990 | 1687K| | 213 (1)| 00:00:03 |

| 3 | TABLE ACCESS FULL| DAVE | 72520 | 2124K| | 213 (1)| 00:00:03 |

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

Predicate Information (identified by operation id):

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

1 -access("D1"."OBJECT_ID"="D2"."OBJECT_ID")

统计信息

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

1 recursive calls

0 db block gets

6353 consistent gets

0 physical reads

0 redo size

3388939 bytes sent via SQL*Net toclient

53874 bytes received via SQL*Netfrom client

4852 SQL*Net roundtrips to/fromclient

0 sorts (memory)

0 sorts (disk)

72762 rows processed

这里的物理读为0. 但是还是走的是全表扫描。

--刷新一下buffer,增加索引条件:

SQL> alter system flush buffer_cache;

系统已更改。

SQL> select d1.object_name,d2.object_type fromdave d1,dave2 d2 where d1.object_id=d2.object_id and d1.object_id <100;

已选择98行。

执行计划

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

Plan hash value: 504164237

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

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

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

| 0 |SELECT STATEMENT | | 3600 | 189K| 23 (5)| 00:00:01 |

|* 1 | HASH JOIN | | 3600 | 189K| 23 (5)| 00:00:01 |

| 2 | TABLE ACCESS BY INDEX ROWID| DAVE2 | 3600 | 86400 | 11 (0)| 00:00:01 |

|* 3 | INDEX RANGE SCAN | IDX_DAVE_OBJECT_ID2 | 648 | | 3 (0)| 00:00:01 |

| 4 | TABLE ACCESS BY INDEX ROWID| DAVE | 3626 | 106K| 11 (0)| 00:00:01 |

|* 5 | INDEX RANGE SCAN | IDX_DAVE_OBJECT_ID | 653| | 3 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

1 -access("D1"."OBJECT_ID"="D2"."OBJECT_ID")

3 -access("D2"."OBJECT_ID"<100)

5 -access("D1"."OBJECT_ID"<100)

统计信息

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

1 recursive calls

0 db block gets

20 consistent gets

6 physical reads

0 redo size

3317 bytes sent via SQL*Net toclient

590 bytes received via SQL*Netfrom client

8 SQL*Net roundtrips to/fromclient

0 sorts (memory)

0 sorts (disk)

98 rows processed

SQL>

走索引之后,物理读从1558降到6.

二.说明

在上面的测试中,我们看到了索引扫描的类型和多表关联的类型,关于这几种类型的说明,参考:

Oracle 索引扫描的五种类型

http://blog.csdn.net/tianlesoftware/article/details/5852106

多表连接的三种方式详解 HASH JOIN MERGE JOINNESTED LOOP

http://blog.csdn.net/tianlesoftware/article/details/5826546

从执行计划中,当我们走索引之后,在对应的表上就会出现:

TABLE ACCESS BY INDEX ROWID

在如下文章中对OracleROWID 有说明

Oracle Rowid 介绍

http://blog.csdn.net/tianlesoftware/article/details/5020718

rowid是伪列(pseudocolumn),在查询结果输出时它被构造出来的。rowid并不会真正存在于表的data block中,其存在于index当中,用来通过rowid来寻找表中的行数据。

ROWID 由以下几部分组成:

1. 数据对象编号:每个数据对象(如表或索引)在创建时都分配有此编号,并且此编号在数据库中是唯一的

2. 相关文件编号:此编号对于表空间中的每个数据文件是唯一的

3. 块编号:表示包含此行的块在数据文件中的位置

4. 行编号:标识块头中行目录位置的位置

Oracle 索引中保存的是我们字段的值和该值对应的rowid,我们根据索引进行查找时,就会返回该block的rowid,然后根据rowid直接去block上去我们需要的数据,因此就出现了:

TABLE ACCESS BY INDEX ROWID

因为ROWID 对应一个block,所以当使用TABLE ACCESS BY INDEX ROWID时,每次就只能读取一个block。

假设我们我们的数据返回100个ROWID,其中10个row 位于同一个block上,那么我们只需要访问91次block,就可以拿到我们需要的数据。

关于如何确定row记录在哪个block的方法参考:

Oracle rdba和 dba 说明

http://blog.csdn.net/tianlesoftware/article/details/6529346

小结:

(1) TABLE ACCESS BY INDEX ROWID 只出现在使用索引的情况下。

(2) TABLE ACCESS BY INDEX ROWID 是单块读,每次只能读取一个block。

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