The ASH contains recent information on active sessions sampled every second. The AWR takes snapshots of the database every hour, so the information in the AWR could be almost an hour old and will not help in diagnosing issues that are current on the database.

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Typically, to resolve issues currently on the database, detailed information pertaining to the last 5 or 10 minutes is critical. Because recording session activity is expensive, ASH samples V$SESSION every second and records the events for which the sessions are waiting.ASH is designed as a rolling buffer in memory; old information is overwritten after saving it to the AWR. You can query ASH information using the V$ACTIVE_SESSION_HISTORY view.The view contains one row for each active session per sample and returns the latest session’s sample rows first.

The AWR is a collection of persistent system performance statistics owned by the SYS schema.Over time, you should purge the statistics, and sometimes you may want to get a snapshot of the system for performance diagnosing outside the regular interval. Oracle 10g provides several programs in a package named DBMS_WORKLOAD_REPOSITORY. Using these programs, you can manage the snapshots and perform baselines.

下面通过调用dbms_workload_repository包中的create_snapshot来手工产生两个快照：
EXEC dbms_workload_repository.create_snapshot();
。。。 。。。 。。。
EXEC dbms_workload_repository.create_snapshot();

紧接着通过awrrpt.sql来产生报告（非常类似statspack中的spreport.sql）
You can view AWR reports using the awrrpt.sql and awrrpti.sql scripts located in the
$ORACLE_HOME/rdbms/admin directory. The awrrpt.sql script displays statistics for a range
of snapshot IDs. The report can be saved as text file or HTML file. The awrrpti.sql script is
similar to awrrpt.sql; the only difference is you can specify the database ID and instance ID
as parameters. The report contains the following categories of information:
 Report summary
 Wait events statistics
 SQL statistics
 Instance activity statistics
 I/O statistics
 Buffer pool statistics
 Advisory statistics
 Wait statistics
 Undo statistics
 Latch statistics
 Segment statistics
 Dictionary cache statistics
 Library cache statistics
 SGA statistics
 Resource limit statistics
 init.ora parameters