[bbk5333] 第80集 第10章 SQL Performance 01

AWR能够帮助DBA识别占用资源最多的SQL语句.

SQL Tune Advisor执行步骤

1. 捕获到占用资源较多的SQL
2. Advisor对捕获到的、占用资源较多的SQL进行分析
3. Advisor产生建议的SQL
4. 执行实施SQL

SQL Tuning Advisor:Overview

Automatic SQL Tuning:Overview

• Automatic SQL Tuning automates the entire SQL tuning process and replaces manual SQL tuning.
• Optimizer modes:
  • Normal modes:
  • Tuning mode or Automatic Tuning Optimizer(ATO)
• SQL Tuning Advisor is used to access tuning mode.
• You should use tuning mode only for high-load SQL statements.

Application Tuning Challenges

Stale or Missing Object Statistics

• Object statistics are key inputs to the optimizer
• ATO verifies object statistics for each query object.
• ATO uses dynamic sampling and generates:
  • -Auxiliary object statistics to compensate for missing or stale object statistics
  • -Recommendations to gather object statistics where appropriate:

DBMS_STATS.GAHTER_TABLE_STATS(

ownname=>'SH',tablename=>'CUSTOMERS',estimate_percent=>DBMS_STATS.AUTO_SAMPLE_SIZE

);

SQL Statement Profiling

• Statement statistics are key inputs to the optimizer.
• ATO verifies statements statistics such as :
  • Predicate selectivity
  • Optimizer settings(FIRST ROWS versus ALL_ROWS)
• Automatic Tuning Optimizer uses
  • Dynamic samling
  • Partial execution of the statement
  • Past execution history statistics of the statment
• ATO builds a profile if statistics were generated:

EXEC :profile_name := DBMS_SQLTUNE.accept_sql_profile(task_name => 'my_sql_tuning_task');

Plan Tuning Flow and SQL Profile Creation

Using the SQL Tuning Advisor

• Use the SQL Tuning Advisor to analyze SQL statements and obtain performance recommendations.
• Sources for SQL Tuning Advisor to analyze:
  • -Top Activity:Analyzes the top SQL statements currently active.
  • -SQL Tuning Sets:Analyzes a set of SQL statements you provide.
  • -Historical SQL(AWR):Analyzes SQL statements from statements collected by AWR snapshots.