某客戶通過監控發現某一套Oracle DB在一個時間段內的，異常指標！需要進行分析
使用如下SQL進行診斷，發現TOP EVENT 是SQL*Net break/reset to client
SQL>select instance_number,event,count(*) from dba_hist_active_sess_history
where SAMPLE_TIME between to_date('2020-02-25 06','yyyy-mm-dd hh24') and
to_date('2020-02-25 09','yyyy-mm-dd hh24') and
having(count(*))>200 group by instance_number,event order by 2;
查詢11g官方文檔獲取該等待事件解釋說明，基本上與應用執行的程序=SQL有關系
SQL*Net break/reset to client (%) Description The server is sending a break or reset message to the client. The session running on the server is waiting for a reply
from the client. These waits are caused by an application attempting to: Select from a closed cursor Select on a cursor after the last row has already been fetched and no data has been returned Select on a non-existent table Insert a duplicate row into a uniquely indexed table Issuing a query with invalid syntax If the value, v$session_wait.p2, for this parameter equals 0, it means a reset was sent to the client. A non-zero value
means that the break was sent to the client.
使用如下SQL 可以查詢到執行SQL的用戶ID=DBA_USER USER_ID=>USERNAME 以及SQL_id
SQL>select event,USER_ID,sql_id,count(*) from dba_hist_active_sess_history where SAMPLE_TIME
between to_date('2020-02-25 06','yyyy-mm-dd hh24') and to_date('2020-02-25 09','yyyy-mm-dd hh24')
and instance_number=1 and event='SQL*Net break/reset to client' group by USER_ID,event,sql_id order by 2;
數據庫並未找到SQL文本，但是該庫存在監控，通過監控輸入SQL_ID找到SQL_TEXT 發現是查詢視圖！
由於本次問題，開發人員自己處理了，因此並未造成影響。開發也並未反饋是什么原因。如下進行測試模擬參考文檔！
https://tanelpoder.com/2008/04/10/sqlnet-breakreset-to-client/
如下是測試內容
SQL> select event, total_waits from v$session_event where event like '%reset%' and sid = (select sid from v$mystat where rownum = 1); SQL> begin execute immediate 'drop table non_existent'; exception when others then null; end; / SQL> select event, total_waits from v$session_event where event like '%reset%' and sid = (select sid from v$mystat where rownum = 1); null
上述可以發現，當查詢一個不存在的表，但是PLSQL存在異常處理部分，並不會出現本次測試的等待事件！
SQL> begin execute immediate 'drop table non_existent'; end; / SQL> select event, total_waits from v$session_event where event like '%reset%' and sid = (select sid from v$mystat where rownum = 1); EVENT TOTAL_WAITS ------------------------------ ----------- SQL*Net break/reset to client 2
上述可以發現，當查詢一個不存在的表，但是PLSQL不存在異常處理部分，該等待事件出現！
查詢一個不存在的表！
SQL> select * from non_existent; select * from non_existent * ERROR at line 1: ORA-00942: table or view does not exist SQL> select event, total_waits from v$session_event where event like '%reset%' and sid = (select sid from v$mystat where rownum = 1); EVENT TOTAL_WAITS ------------------------------ ----------- SQL*Net break/reset to client 4 創建一個表，后置查詢一個不存在的表！ SQL> create table a as select * from employees; create table a as select * from employees * ERROR at line 1: ORA-00942: table or view does not exist SQL> select event, total_waits from v$session_event where event like '%reset%' and sid = (select sid from v$mystat where rownum = 1); EVENT TOTAL_WAITS ------------------------------ ----------- SQL*Net break/reset to client 6 查詢一個視圖，視圖對應的表被刪除 SQL> create table a as select * from hr.employees; SQL> create view a_t as select EMPLOYEE_ID,FIRST_NAME from a; SQL> drop table a; SQL> select event, total_waits from v$session_event where event like '%reset%' and sid = (select sid from v$mystat where rownum = 1); EVENT TOTAL_WAITS ------------------------------ ----------- SQL*Net break/reset to client 6 SQL> select * from a_t; select * from a_t * ERROR at line 1: ORA-04063: view "SYS.A_T" has errors SQL> select event, total_waits from v$session_event where event like '%reset%' and sid = (select sid from v$mystat where rownum = 1); EVENT TOTAL_WAITS ------------------------------ ----------- SQL*Net break/reset to client 8