Oracle 11g新特性之收集多列统计信息 - 数据库编程

我们在写SQL语句的时候，经常会碰到where子句后面有多个条件的情况，也就是根据多列的条件筛选得到数据。默认情况下，oracle会把多列的选择率（selectivity）相乘从而得到where语句的选择率，这样有可能造成选择率（selectivity）不准确，从而导致优化器做出错误的判断。为了能够让优化器做出准确的判断，从而生成准确的执行计划，oracle在11g数据库中引入了收集多列统计信息。本文通过对测试表的多条件查询，介绍收集多列统计信息的重要性。

1.环境准备

我们在Oracle 11g中进行试验。

SQL>
SQL> select * from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - Production
PL/SQL Release 11.2.0.3.0 - Production
CORE 11.2.0.3.0 Production
TNS for Linux: Version 11.2.0.3.0 - Production
NLSRTL Version 11.2.0.3.0 - Production
SQL>

在hr用户下创建测试表hoegh，重复插入数据，数据量相当于16个employees表（总行数1712＝107*16）。

SQL>
SQL> conn hr/hr
Connected.
SQL>
SQL> create table hoegh as select * from employees;
Table created.
SQL> select count(*) from hoegh;
COUNT(*)
----------
107
SQL>
SQL> insert into hoegh select * from hoegh;
107 rows created.
SQL> /
214 rows created.
SQL> /
428 rows created.
SQL> /
856 rows created.
SQL> commit;
Commit complete.
SQL> select count(*) from hoegh;
COUNT(*)
----------
1712
SQL>

2.按照常规方法收集统计量信息；

SQL>

SQL> exec dbms_stats.gather_table_stats(\'HR\',\'HOEGH\');

PL/SQL procedure successfully completed.

SQL>

3.查看执行单个条件的where语句的执行计划

SQL>
?
SQL> explain plan for select * from hoegh where employee_id=110;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT

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

Plan hash value: 774871165

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

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

| 0 | SELECT STATEMENT | | 16 | 1104 | 8 (0)| 00:00:01 |

|* 1 | TABLE ACCESS FULL| HOEGH | 16 | 1104 | 8 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

PLAN_TABLE_OUTPUT

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

? 1 - filter(\"EMPLOYEE_ID\"=110)

13 rows selected.

SQL>
从执行计划可以看出返回了16行记录，结果没有问题。可是，这个16是哪儿来的呢，我们先要了解选择率（selectivity）和返回行数是如何计算的：
选择率（selectivity）＝在本例中是 1/唯一值
返回行数＝选择率（selectivity）＊表记录总数

?也就是说，在这个查询语句中，选择率＝1/107，返回行数＝1/107*1712=16

4.查看执行两个条件的where语句的执行计划

SQL>
?
SQL> explain plan for select * from hoegh where employee_id=110 and email=\'JCHEN\';

Explained.

SQL>

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT

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

Plan hash value: 774871165

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

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

| 0 | SELECT STATEMENT | | 1 | 69 | 8 (0)| 00:00:01 |

|* 1 | TABLE ACCESS FULL| HOEGH | 1 | 69 | 8 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

PLAN_TABLE_OUTPUT

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

? 1 - filter(\"EMPLOYEE_ID\"=110 AND \"EMAIL\"=\'JCHEN\')

13 rows selected.

Oracle 11g新特性之收集多列统计信息(一)