[20201204]為什么回傳2行記錄.txt
--//在itpub問的問題，http://www.itpub.net/thread-2140116-1-1.html，花了一點點時間，大概知道問題在哪里，
--//問題出在一致性讀取上，參考鏈接http://blog.itpub.net/267265/viewspace-2213824/ =>[20180907]訪問v$視圖與一致性讀取.txt
1.問題提出：
SCOTT@book> @ ver1
PORT_STRING VERSION BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx 11.2.0.4.0 Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
SCOTT@book> select sid from v$mystat where rownum=1;
SID
----------
32
SCOTT@book> select sid ,osuser from v$session where sid = 32;
SID OSUSER
---------- ------------------------------
32 oracle
--//回傳1行.如果加入提示，
SCOTT@book> alter session set statistics_level = all;
Session altered.
SCOTT@book> select /*+ leading(s e w) */ sid ,osuser from v$session where sid = 32;
SID OSUSER
---------- ------------------------------
32 oracle
32 oracle
--//查看執行計劃：
SCOTT@book> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 41djy6556s87h, child number 1
-------------------------------------
select /*+ leading(s e w) */ sid ,osuser from v$session where sid = 32
Plan hash value: 1247622051
---------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| A-Rows | A-Time | OMem | 1Mem | Used-Mem |
---------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 1 (100)| 2 |00:00:00.01 | | | |
| 1 | NESTED LOOPS | | 1 | 1 | 108 | 0 (0)| 2 |00:00:00.01 | | | |
| 2 | MERGE JOIN CARTESIAN | | 1 | 1 | 82 | 0 (0)| 1367 |00:00:00.01 | | | |
|* 3 | FIXED TABLE FIXED INDEX| X$KSUSE (ind:1) | 1 | 1 | 69 | 0 (0)| 1 |00:00:00.01 | | | |
| 4 | BUFFER SORT | | 1 | 100 | 1300 | 0 (0)| 1367 |00:00:00.01 | 50176 | 50176 |45056 (0)|
| 5 | FIXED TABLE FULL | X$KSLED | 1 | 100 | 1300 | 0 (0)| 1367 |00:00:00.01 | | | |
|* 6 | FIXED TABLE FIXED INDEX | X$KSLWT (ind:1) | 1367 | 1 | 26 | 0 (0)| 2 |00:00:00.01 | | | |
---------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$88122447
3 - SEL$88122447 / S@SEL$4
5 - SEL$88122447 / E@SEL$4
6 - SEL$88122447 / W@SEL$4
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter(("S"."INDX"=32 AND "S"."INST_ID"=USERENV('INSTANCE') AND BITAND("S"."KSSPAFLG",1)<>0 AND BITAND("S"."KSUSEFLG",1)<>0))
6 - filter(("W"."KSLWTSID"=32 AND "W"."KSLWTEVT"="E"."INDX"))
--//你可以發現id=6,A-Row確實回傳2行，為什么呢？
2.繼續探究：
--//打開新的會話，原來會話不退出，
SCOTT@book> select /*+ leading(s e w) */ sid ,osuser from v$session where sid = 32;
SID OSUSER
---------- ------------------------------
32 oracle
--//相同的提示僅僅回傳一行，再問為什么呢？
--//先猜測一下X$KSLWT，從名字上可以猜測是等待事件相關的視圖，顯示多加入幾個欄位：
SCOTT@book> select /*+ leading(s e w) */ sid ,osuser,EVENT#,event from v$session where sid = 32;
SID OSUSER EVENT# EVENT
---------- ------------------------------ ---------- ----------------------------------------
32 oracle 354 SQL*Net message from client
--//回到原來的會話，重復執行：
SCOTT@book> select /*+ leading(s e w) */ sid ,osuser,EVENT#,event from v$session where sid = 32;
SID OSUSER EVENT# EVENT
---------- ------------------------------ ---------- ----------------------------------------
32 oracle 350 SQL*Net message to client
32 oracle 354 SQL*Net message from client
--//這樣就很好決議為什么回傳2行了，視圖不能保證一致性，
--//該陳述句執行時先出現SQL*Net message to client等待，然后是SQL*Net message from client等待，
--//由于視圖一致性無法保證，就出現回傳2行的情況，而另外的會話在不會出現這樣的情況，
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標籤：Oracle