[bbk4949]第64集 第7章 数据库的维护 01

Oracle Optimizer:Overview

The Oracle optimizer determines the most efficient execution plan and is the most important step in the processing of any SQL statement.

The Optimizer

• Evaluates expressions and conditions
• Uses object and system statistics
• Decides how to access the data
• Decides how to join tables
• Determines the most efficient path

Optimizer Statistics

Optimizer statistics are:

• A snapshot at a point in time
• Persistent across instance restarts
• Collected automatically

SQL> select count(*) from frx_oa_dict;

  COUNT(*)
----------
       480

SQL> select num_rows from dba_tables
  2  where owner='ARCER' and table_name = 'FRX_OA_DICT';

  NUM_ROWS
----------
       479

上述SQL语句说明，优化器的统计数据与实际表中的数据，有可能存在差距的.而oracle判断执行计划的依据则是数据字典中的统计数据.

每个语句就是一个游标.

Using the Manage Optimizer Statistics Page

Gathering Optimizer Statistics Manually

Preferences(首选项) for Gathering Statistics