【书评:Oracle查询优化改写】第三章

【书评:Oracle查询优化改写】第三章

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导读

各位技术爱好者，看完本文后，你可以掌握如下的技能，也可以学到一些其它你所不知道的知识，~O(∩_∩)O~：

① 隐含参数 _b_tree_bitmap_plans介绍

② 11g新特性Native Full Outer Join

本文如有错误或不完善的地方请大家多多指正，ITPUB留言或QQ皆可，您的批评指正是我写作的最大动力。

实验环境介绍

oracle：11.2.0.3 、8.1.7.0.0

OS: RHEL6.5

前言

前2章的链接参考相关连接：

【书评:Oracle查询优化改写】第一章 http://blog.itpub.net/26736162/viewspace-1652985/

【书评:Oracle查询优化改写】第二章http://blog.itpub.net/26736162/viewspace-1654252/

昨天晚上(5.14)看完了《Oracle查询优化改写》的第三章，不得不说下这本书里边代码的排版有很大问题，格式老是不对齐，尤其是执行计划的格式，可能是印刷的时候出现的问题吧，不说这个了。这个第三章主要是讲多表的关联，包括各种连接的写法，如左联、右联，以及过滤条件错误地放在WHERE里会有什么影响；当数据有重复值时要直接关联还是分组汇总后再关联。

第 3 章操作多个表

3.1 UNION ALL 与空字符串

3.2 UNION 与 OR

3.3 组合相关的行

3.4 IN、EXISTS 和 INNER JOIN

3.5 INNER JOIN、LEFT JOIN、RIGHT JOIN 和 FULL JOIN 解析

3.6 自关联

3.7 NOT IN、NOT EXISTS 和 LEFT JOIN

3.8 外连接中的条件不要乱放

3.9 检测两个表中的数据及对应数据的条数是否相同

3.10 聚集与内连接

3.11 聚集与外连接

3.12 从多个表中返回丢失的数据

3.13 多表查询时的空值处理

下边我就针对一些重点，或者说是我自己也不是很懂的部分做做研究吧。

隐含参数 _b_tree_bitmap_plans 实验

简介

该参数为隐含参数，是指是否将索引转换为bitmap索引然后执行，在oracle9i之前默认值为false，之后的默认值为true。可以这样认为，如有两个字段A，B都有btree索引，oracle有可能将这两个索引转换成bitmap索引然后做and操作得出结果集。如果改为false就会选用其中的一个索引，走btree的索引，我们可以将该参数在session或系统级别设置为false，也可以加hint /*+ opt_param('_b_tree_bitmap_plans', 'false') */ 来实现禁用该参数。

 symptom: Execution plan operation shows bitmap conversion from rowids

 symptom: No bitmap indexes

 symptom: Execution plan shows BITMAP CONVERSION

 cause: In 7.3.4 and in 8.1.7 default value of _b_tree_bitmap_plans is FALSE
whereas as of 9.0.1 (and 9.2) the default value is TRUE When _b_tree_bitmap_plans set to true (advice not to change the default setting
yourself) the optimizer is allowed to produce bitmap plans for normal b*tree
indexes even if no bitmap indexes set.

相关的执行计划中可能转换为如下的形式：

（1）BITMAP CONVERSION FROM ROWIDS

将一批数据记录的ROWID映射为位图。

对于普通B*树索引，Oracle也可以将数据记录的ROWID映射成一个位图，然后进行位图操作。进行这样的转换需要将系统参数_b_tree_bitmap_plans设置为TRUE。

（2）BITMAP CONVERSION TO ROWIDS

将位图映射为ROWID。在一个位图键值中，包含了一批数据记录的起始地址和结束地址，且这批记录是连续的，因此位图中的每一个位就按序对应了一条数据记录。

（3）BITMAP OR

对位图进行"或"（OR）操作。在查询的过滤条件中，如果位图索引字段直接的关系是"或"，可以通过BITMAP OR来判断位图所映射的一批数据记录是否满足条件。

eygle大师的一个例子：

http://www.eygle.com/archives/2011/12/bitmap_conversion_cpu.html

11g情况下

[root@rhel6_lhr ~]# su - oracle

[oracle@rhel6_lhr ~]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.3.0 Production on Fri May 10:16:10 2015

Connected to:

Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production

With the Partitioning, Automatic Storage Management, OLAP, Data Mining

and Real Application Testing options

10:16:10 SQL>

10:16:10 SQL> conn lhr/lhr

Connected.

10:16:10 SQL> create table emp_bk as select * from scott.emp;

Table created.

Elapsed: 00:00:03.43

10:16:15 SQL> create index idx_emp_empno on emp_bk(empno);

Index created.

Elapsed: 00:00:00.05

10:19:26 SQL> create index idx_emp_ename on emp_bk(ename);

Index created.

Elapsed: 00:00:00.04

10:20:48 SQL> explain plan for select empno,ename from emp_bk where empno=7788 or ename='SCOTT';

Explained.

Elapsed: 00:00:00.09

10:20:56 SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT

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

Plan hash value: 4193090541

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

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

| 0 | SELECT STATEMENT | | 1 | 20 | 2 (0)| 00:00:01 |

| 1 | TABLE ACCESS BY INDEX ROWID | EMP_BK | 1 | 20 | 2 (0)| 00:00:01 |

| 2 | BITMAP CONVERSION TO ROWIDS | | | | | |

| 3 | BITMAP OR | | | | | |

| 4 | BITMAP CONVERSION FROM ROWIDS| | | | | |

|* 5 | INDEX RANGE SCAN | IDX_EMP_EMPNO | | | 1 (0)| 00:00:01 |

| 6 | BITMAP CONVERSION FROM ROWIDS| | | | | |

|* 7 | INDEX RANGE SCAN | IDX_EMP_ENAME | | | 1 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

5 - access("EMPNO"=7788)

7 - access("ENAME"='SCOTT')

Note

-----

- dynamic sampling used for this statement (level=2)

24 rows selected.

Elapsed: 00:00:00.52

10:24:06 SQL> conn / as sysdba

Connected.

Elapsed: 00:00:00.03

10:24:34 SQL> set pagesize 9999

10:24:41 SQL> set line 9999

10:24:41 SQL> col NAME format a30

10:24:41 SQL> col KSPPDESC format a50

10:24:41 SQL> col KSPPSTVL format a20

10:24:42 SQL> SELECT a.INDX,

10:24:42 2 a.KSPPINM NAME,

10:24:42 3 a.KSPPDESC,

10:24:42 4 b.KSPPSTVL

10:24:42 5 FROM x$ksppi a,

10:24:42 6 x$ksppcv b

10:24:42 7 WHERE a.INDX = b.INDX

10:24:42 8 and lower(a.KSPPINM) like lower('%&parameter%');

Enter value for parameter: _b_tree_bitmap_plans

old 8: and lower(a.KSPPINM) like lower('%&parameter%')

new 8: and lower(a.KSPPINM) like lower('%_b_tree_bitmap_plans%')

INDX NAME KSPPDESC KSPPSTVL

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

1910 _b_tree_bitmap_plans enable the use of bitmap plans for tables w. only TRUE

B-tree indexes

Elapsed: 00:00:00.01

10:25:44 SQL> conn lhr/lhr

Connected.

10:26:56 SQL> alter session set "_b_tree_bitmap_plans" = false;

Session altered.

Elapsed: 00:00:00.00

10:27:01 SQL> show parameter _b_tree_bitmap_plans

NAME TYPE VALUE

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

_b_tree_bitmap_plans boolean FALSE

10:27:05 SQL> explain plan for select empno,ename from emp_bk where empno=7788 or ename='SCOTT';

Explained.

Elapsed: 00:00:00.01

10:27:14 SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT

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

Plan hash value: 370270337

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

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

| 0 | SELECT STATEMENT | | 1 | 20 | 3 (0)| 00:00:01 |

|* 1 | TABLE ACCESS FULL| EMP_BK | 1 | 20 | 3 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

1 - filter("EMPNO"=7788 OR "ENAME"='SCOTT')

Note

-----

- dynamic sampling used for this statement (level=2)

17 rows selected.

Elapsed: 00:00:00.04

10:27:18 SQL> explain plan for select empno,ename from emp_bk where empno=7788

10:27:49 2 union

10:27:55 3 select empno,ename from emp_bk where ename='SCOTT';

Explained.

Elapsed: 00:00:00.00

10:28:07 SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT

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

Plan hash value: 3014579657

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

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

| 0 | SELECT STATEMENT | | 2 | 40 | 6 (67)| 00:00:01 |

| 1 | SORT UNIQUE | | 2 | 40 | 6 (67)| 00:00:01 |

| 2 | UNION-ALL | | | | | |

| 3 | TABLE ACCESS BY INDEX ROWID| EMP_BK | 1 | 20 | 2 (0)| 00:00:01 |

|* 4 | INDEX RANGE SCAN | IDX_EMP_EMPNO | 1 | | 1 (0)| 00:00:01 |

| 5 | TABLE ACCESS BY INDEX ROWID| EMP_BK | 1 | 20 | 2 (0)| 00:00:01 |

|* 6 | INDEX RANGE SCAN | IDX_EMP_ENAME | 1 | | 1 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

4 - access("EMPNO"=7788)

6 - access("ENAME"='SCOTT')

Note

-----

- dynamic sampling used for this statement (level=2)

23 rows selected.

Elapsed: 00:00:00.01

10:28:13 SQL> Select Name ,Value From v$parameter Where Name ='_b_tree_bitmap_plans' ;

NAME VALUE

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

_b_tree_bitmap_plans FALSE

Elapsed: 00:00:00.02

10:34:06 SQL> alter session set "_b_tree_bitmap_plans" = true;

Session altered.

Elapsed: 00:00:00.00

11:19:04 SQL> explain plan for select /*+ opt_param('_b_tree_bitmap_plans', 'false') */ empno,ename from emp_bk where empno=7788 or ename='SCOTT';

Explained.

Elapsed: 00:00:00.08

11:19:22 SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT

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

Plan hash value: 370270337

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

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

| 0 | SELECT STATEMENT | | 1 | 20 | 3 (0)| 00:00:01 |

|* 1 | TABLE ACCESS FULL| EMP_BK | 1 | 20 | 3 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

1 - filter("EMPNO"=7788 OR "ENAME"='SCOTT')

Note

-----

- dynamic sampling used for this statement (level=2)

17 rows selected.

Elapsed: 00:00:00.24

由实验可以看出，_b_tree_bitmap_plans设置为false后，emp_bk走了全表扫描，并没有走位图索引转换。

8i情况下

C:UsersAdministrator>sqlplus "lhr/lhr@orcl8i as sysdba"

SQL*Plus: Release 8.1.7.0.0 - Production on Mon May 18 10:44:28 2015

Connected to:

Oracle8i Enterprise Edition Release 8.1.7.0.0 - Production

With the Partitioning option

JServer Release 8.1.7.0.0 - Production

SQL> set pagesize 9999

SQL> set line 9999

SQL> col NAME format a30

SQL> col KSPPDESC format a50

SQL> col KSPPSTVL format a20

SQL> SELECT a.INDX,

2 a.KSPPINM NAME,

3 a.KSPPDESC,

4 b.KSPPSTVL

5 FROM x$ksppi a,

6 x$ksppcv b

7 WHERE a.INDX = b.INDX

8 and lower(a.KSPPINM) like lower('%&parameter%');

Enter value for parameter: _b_tree_bitmap_plans

old 8: and lower(a.KSPPINM) like lower('%&parameter%')

new 8: and lower(a.KSPPINM) like lower('%_b_tree_bitmap_plans%')

INDX NAME KSPPDESC KSPPSTVL

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

348 _b_tree_bitmap_plans enable the use of bitmap plans for tables w. only FALSE

B-tree indexes

SQL>

SQL> create table lhr.emp_bk as select * from scott.emp;

Table created.

SQL> create index lhr.idx_emp_empno on lhr.emp_bk(empno);

Index created.

SQL> create index lhr.idx_emp_ename on lhr.emp_bk(ename);

Index created.

SQL> set line 9999 pagesize 9999

SQL> set autot on;

SQL> select empno,ename from lhr.emp_bk where empno=7788 or ename='SCOTT';

EMPNO ENAME

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

7788 SCOTT

Execution Plan

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

0 SELECT STATEMENT Optimizer=CHOOSE

1 0 CONCATENATION

2 1 TABLE ACCESS (BY INDEX ROWID) OF 'EMP_BK'

3 2 INDEX (RANGE SCAN) OF 'IDX_EMP_ENAME' (NON-UNIQUE)

4 1 TABLE ACCESS (BY INDEX ROWID) OF 'EMP_BK'

5 4 INDEX (RANGE SCAN) OF 'IDX_EMP_EMPNO' (NON-UNIQUE)

Statistics

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

0 recursive calls

0 db block gets

0 consistent gets

0 physical reads

0 redo size

0 bytes sent via SQL*Net to client

0 bytes received via SQL*Net from client

0 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

SQL>

8i下默认为false，执行计划也完全不同。

Native Full Outer Join

关于这个特性可以参考如下文章：

http://blog.itpub.net/26736162/viewspace-1660038/

我们在10.2.0.4下测试一下：

[oracle@rhel6_lhr ~]$ sqlplus / as sysdba

SQL*Plus: Release 10.2.0.4.0 - Production on Mon May 18 11:41:13 2015

Connected to an idle instance.

SQL> startup

ORACLE instance started.

Total System Global Area 448790528 bytes

Fixed Size 2084616 bytes

Variable Size 121635064 bytes

Database Buffers 318767104 bytes

Redo Buffers 6303744 bytes

Database mounted.

Database opened.

SQL> create table lhr.emp_bk as select * from scott.emp;

Table created.

SQL> create table lhr.emp_bk as select * from scott.emp;

Table created.

SQL> set autot on;

SQL> set line 9999 pagesize 9999

SQL> select * from lhr.emp_bk a full outer join lhr.emp_bk2 b on a.empno=b.empno;

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO

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

7369 SMITH CLERK 7902 1980-12-17 00:00:00 800 20 7369 SMITH CLERK 7902 1980-12-17 00:00:00 800 20

7499 ALLEN SALESMAN 7698 1981-02-20 00:00:00 1600 300 30 7499 ALLEN SALESMAN 7698 1981-02-20 00:00:00 1600 300 30

7521 WARD SALESMAN 7698 1981-02-22 00:00:00 1250 500 30 7521 WARD SALESMAN 7698 1981-02-22 00:00:00 1250 500 30

7566 JONES MANAGER 7839 1981-04-02 00:00:00 2975 20 7566 JONES MANAGER 7839 1981-04-02 00:00:00 2975 20

7654 MARTIN SALESMAN 7698 1981-09-28 00:00:00 1250 1400 30 7654 MARTIN SALESMAN 7698 1981-09-28 00:00:00 1250 1400 30

7698 BLAKE MANAGER 7839 1981-05-01 00:00:00 2850 30 7698 BLAKE MANAGER 7839 1981-05-01 00:00:00 2850 30

7782 CLARK MANAGER 7839 1981-06-09 00:00:00 2450 10 7782 CLARK MANAGER 7839 1981-06-09 00:00:00 2450 10

7788 SCOTT ANALYST 7566 1987-04-19 00:00:00 3000 20 7788 SCOTT ANALYST 7566 1987-04-19 00:00:00 3000 20

7839 KING PRESIDENT 1981-11-17 00:00:00 5000 10 7839 KING PRESIDENT 1981-11-17 00:00:00 5000 10

7844 TURNER SALESMAN 7698 1981-09-08 00:00:00 1500 0 30 7844 TURNER SALESMAN 7698 1981-09-08 00:00:00 1500 0 30

7876 ADAMS CLERK 7788 1987-05-23 00:00:00 1100 20 7876 ADAMS CLERK 7788 1987-05-23 00:00:00 1100 20

7900 JAMES CLERK 7698 1981-12-03 00:00:00 950 30 7900 JAMES CLERK 7698 1981-12-03 00:00:00 950 30

7902 FORD ANALYST 7566 1981-12-03 00:00:00 3000 20 7902 FORD ANALYST 7566 1981-12-03 00:00:00 3000 20

7934 MILLER CLERK 7782 1982-01-23 00:00:00 1300 10 7934 MILLER CLERK 7782 1982-01-23 00:00:00 1300 10

14 rows selected.

Execution Plan

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

Plan hash value: 914601651

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

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

| 0 | SELECT STATEMENT | | 15 | 2610 | 13 (8)| 00:00:01 |

| 1 | VIEW | | 15 | 2610 | 13 (8)| 00:00:01 |

| 2 | UNION-ALL | | | | | |

|* 3 | HASH JOIN OUTER | | 14 | 2436 | 7 (15)| 00:00:01 |

| 4 | TABLE ACCESS FULL| EMP_BK | 14 | 1218 | 3 (0)| 00:00:01 |

| 5 | TABLE ACCESS FULL| EMP_BK2 | 14 | 1218 | 3 (0)| 00:00:01 |

|* 6 | HASH JOIN ANTI | | 1 | 100 | 7 (15)| 00:00:01 |

| 7 | TABLE ACCESS FULL| EMP_BK2 | 14 | 1218 | 3 (0)| 00:00:01 |

| 8 | TABLE ACCESS FULL| EMP_BK | 14 | 182 | 3 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

3 - access("A"."EMPNO"="B"."EMPNO"(+))

6 - access("A"."EMPNO"="B"."EMPNO")

Note

-----

- dynamic sampling used for this statement

Statistics

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

338 recursive calls

0 db block gets

61 consistent gets

6 physical reads

0 redo size

2521 bytes sent via SQL*Net to client

492 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

4 sorts (memory)

0 sorts (disk)

14 rows processed

SQL> select /*+ NATIVE_FULL_OUTER_JOIN */ * from lhr.emp_bk a full outer join lhr.emp_bk2 b on a.empno=b.empno;