Database Maintenance 维护DB
you should be able to:
Manage optimizer statistics
Manage the Automatic Workload Repository (AWR) 自动负载知识库
Use the Automatic Database Diagnostic Monitor (ADDM) 自动数据库诊断监控
Describe and use the advisory framework
Set alert thresholds 阈值
Use server-generated alerts
Use automated tasks
AWR --（严重错误）--> ADR 自动诊断知识库
术语
Automatic Workload Repository (AWR): Infrastructure for data gathering, analysis, and solutions recommendations
AWR Baseline: AWR基线 A set of AWR snapshots for performance comparison
Metric: 度量值 Rate of change in a cumulative statistic
Statistics: 统计信息Data collections providing database and object detail
Optimizer statistics: 优化器统计信息Used by query optimizer
Database statistics: 数据库统计信息Used for performance
Threshold: 阈值A boundary value againstwhich metric values are compared
Oracle Optimizer Oracle 优化器
依赖于统计信息，找出最优执行计划：
1、评估表达式和条件
2、使用统计信息
3、决定如何访问数据
4、多表JOIN连接方式
5、找出最优的路径
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
SQL> SELECT COUNT() FROM hr.employees;
COUNT()
214SQL> SELECT num_rows FROM dba_tables
2 WHERE owner='HR' AND table_name = 'EMPLOYEES';
NUM_ROWS
107
exec dbms_stats.set_table_prefs('SH','SALES','STALE_PERCENT','13');
AWR： 默认每60分钟生成快照，默认保留8天，使用MMON进程
建议使用TYPICAL。
select from DBA_OUTSTANDING_ALERTS;
告警消失后告警信息被放入DBA_ALERT_HISTORY;
select from DBA_ALERT_HISTORY;
有状态告警先进入DBA_OUTSTANDING_ALERT;
无状态告警直接进入DBA_ALERT_HISTORY;