在一次生成AWR报告中，发现在“Instances in this Workload Repository schema”部分，出现了多个实例记录信息（host敏感信息被用host1,host2,host3替换）。具体信息如下截图所示：
SQL> @?/rdbms/admin/awrrpt
Current Instance
~~~~~~~~~~~~~~~~
DB Id DB Name Inst Num Instance
----------- ------------ -------- ------------
3990839260 SCM2 1 SCM2
Specify the Report Type
~~~~~~~~~~~~~~~~~~~~~~~
Would you like an HTML report, or a plain text report?
Enter 'html' for an HTML report, or 'text' for plain text
Defaults to 'html'
Enter value for report_type: html
Type Specified: html
Instances in this Workload Repository schema
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DB Id Inst Num DB Name Instance Host
------------ -------- ------------ ------------ ------------
3990839260 1 SCM2 SCM2 host1
* 3990839260 1 SCM2 SCM2 host2
3990839260 1 SCM2 SCM2 host3
Using 3990839260 for database Id
Using 1 for instance number
Specify the number of days of snapshots to choose from
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Entering the number of days (n) will result in the most recent
(n) days of snapshots being listed. Pressing <return> without
specifying a number lists all completed snapshots.
其实这几个hostname对于我来说，非常熟悉，一个是数据库迁移前的hostname，一个是hostname修改前的名称，一个是hostname修改后的名称。那么为什么出现这个情况呢？我们先从$ORACLE_HOME/rdbms/admin/awrrpt.sql脚本开始，看看这些记录是怎么获取的。
[oracle@MyOracle admin]$ more awrrpt.sql
Rem $Header: awrrpt.sql 24-oct-2003.12:04:53 pbelknap Exp $
Rem
Rem awrrpt.sql
Rem
Rem Copyright (c) 1999, 2003, Oracle Corporation. All rights reserved.
Rem
Rem NAME
Rem awrrpt.sql
Rem
Rem DESCRIPTION
Rem This script defaults the dbid and instance number to that of the
Rem current instance connected-to, then calls awrrpti.sql to produce
Rem the Workload Repository report.
Rem
Rem NOTES
Rem Run as select_catalog privileges.
Rem This report is based on the Statspack report.
Rem
Rem If you want to use this script in an non-interactive fashion,
Rem see the 'customer-customizable report settings' section in
Rem awrrpti.sql
Rem
Rem MODIFIED (MM/DD/YY)
Rem pbelknap 10/24/03 - swrfrpt to awrrpt
Rem pbelknap 10/14/03 - moving params to rpti
Rem pbelknap 10/02/03 - adding non-interactive mode cmnts
Rem mlfeng 09/10/03 - heading on
Rem aime 04/25/03 - aime_going_to_main
Rem mlfeng 01/27/03 - mlfeng_swrf_reporting
Rem mlfeng 01/13/03 - Update comments
Rem mlfeng 07/08/02 - swrf flushing
Rem mlfeng 06/12/02 - Created
Rem
--
-- Get the current database/instance information - this will be used
-- later in the report along with bid, eid to lookup snapshots
set echo off heading on underline on;
column inst_num heading "Inst Num" new_value inst_num format 99999;
column inst_name heading "Instance" new_value inst_name format a12;
column db_name heading "DB Name" new_value db_name format a12;
column dbid heading "DB Id" new_value dbid format 9999999999 just c;
prompt
prompt Current Instance
prompt ~~~~~~~~~~~~~~~~
select d.dbid dbid
, d.name db_name
, i.instance_number inst_num
, i.instance_name inst_name
from v$database d,
v$instance i;
@@awrrpti
undefine num_days;
undefine report_type;
undefine report_name;
undefine begin_snap;
undefine end_snap;
--
-- End of file
如上所示，其实awrrpt.sql里面没有多少料，主要功能还是在awrrpti.sql中实现的。在awrrpti.sql里面，看到通过下面部分信息，定位到AWR里面的信息来源于awrinput.sql
检查awrinput.sql脚本，发现AWR那段数据来自于下面SQL语句
--
-- Request the DB Id and Instance Number, if they are not specified
column instt_num heading "Inst Num" format 99999;
column instt_name heading "Instance" format a12;
column dbb_name heading "DB Name" format a12;
column dbbid heading "DB Id" format a12 just c;
column host heading "Host" format a12;
prompt
prompt
prompt Instances in this Workload Repository schema
prompt ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
select distinct
(case when cd.dbid = wr.dbid and
cd.name = wr.db_name and
ci.instance_number = wr.instance_number and
ci.instance_name = wr.instance_name and
ci.host_name = wr.host_name
then '* '
else ' '
end) || wr.dbid dbbid
, wr.instance_number instt_num
, wr.db_name dbb_name
, wr.instance_name instt_name
, wr.host_name host
from dba_hist_database_instance wr, v$database cd, v$instance ci;
prompt
prompt Using &&dbid for database Id
prompt Using &&inst_num for instance number
最后发现原因是:视图dba_hist_database_instance里面存放的历史记录，记录了host的变化，所以上面SQL，出现了三条记录。
dba_hist_database_instance视图脚本
select dbid, instance_number, startup_time, parallel, version,
db_name, instance_name, host_name, last_ash_sample_id
from WRM$_DATABASE_INSTANCE
使用下面脚本找到对应的记录，然后删除那些历史数据就可以了。但是手工删除这个表的数据是否会有问题呢? 也在测试环境测试了一下，没有任何问题，和同事讨论了一下，觉得这个数据的删除，不会出现什么问题。
SQL> SELECT HOST_NAME, MAX(STARTUP_TIME) FROM dba_hist_database_instance
2 GROUP BY HOST_NAME;
SQL> DELETE FROM dba_hist_database_instance WHERE STARTUP_TIME <=TO_DATE('2016-05-05 00:00:00', 'YYYY-MM-DD HH24:MI:SS');
COMMIT;
266 rows deleted.
SQL>COMMIT;