[20190319]shared pool latch与library cache latch的简单探究.txt

--//昨天看Oracle DBA手记3：数据库性能优化与内部原理解析.pdf 电子书,看了eygle的关于latch之类的测试.
--//自己也重复测试看看.

--//首先说明一下11g已经不存在这个shared pool latch,改为mutexes.所以测试在10g下进行:
--//注意不要在生产系统做这样的测试!!

1.环境:
SCOTT@test> @ &r/ver1
PORT_STRING VERSION BANNER
------------------------------ -------------- ----------------------------------------------------------------
x86_64/Linux 2.4.xx 10.2.0.4.0 Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi

--//把一些常用命令先执行多次(3次以上避免执行时分析时挂起),比如 @ &r/wait10g,也可以事先多开几个sys登录会话.

$ cat wait10g.sql
select p1raw,p2raw,p3raw,p1,p2,p3,sid,serial#,seq#,event,state,wait_time,seconds_in_wait from v$session where wait_class<>'Idle'
and sid not in (select sid from v$mystat where rownum=1)
order by event ;

SELECT addr
        ,latch#
        ,child#
        ,level#
        ,name
        ,gets
        ,sleeps
        ,immediate_gets
        ,immediate_misses
        ,spin_gets
    FROM V$LATCH_CHILDREN
   WHERE name LIKE 'library cache';

ADDR             LATCH# CHILD# LEVEL# NAME          GETS SLEEPS IMMEDIATE_GETS IMMEDIATE_MISSES SPIN_GETS
---------------- ------ ------ ------ ------------- ---- ------ -------------- ---------------- ---------
000000007A753430    215     29      5 library cache 1256      0              0                0         1
000000007A7534D0    215     28      5 library cache 741      0              0                0         0
000000007A753570    215     27      5 library cache 1022      0              0                0         0
000000007A753610    215     26      5 library cache 958      0              0                0         0
000000007A7536B0    215     25      5 library cache 1079      0              0                0         0
000000007A753750    215     24      5 library cache 1022      0              0                0         0
000000007A7537F0    215     23      5 library cache 1076      0              0                0         3
000000007A753890    215     22      5 library cache 950      0              0                0         0
000000007A753930    215     21      5 library cache 1028      0              0                0         0
000000007A7539D0    215     20      5 library cache 1223      0              0                0         0
000000007A753A70    215     19      5 library cache 941      0              0                0         0
000000007A753B10    215     18      5 library cache 781      0              0                0         0
000000007A753BB0    215     17      5 library cache 1050      0              0                0         3
000000007A753C50    215     16      5 library cache 1098      0              0                0         0
000000007A753CF0    215     15      5 library cache 1295      0              0                0         1
000000007A753D90    215     14      5 library cache 1431      0              0                0         1
000000007A753E30    215     13      5 library cache 1186      0              1                0         0
000000007A753ED0    215     12      5 library cache 1265      0              0                0         2
000000007A753F70    215     11      5 library cache 942      0              0                0         0
000000007A754010    215     10      5 library cache 1401      0              0                0         6
000000007A7540B0    215      9      5 library cache 1181      0              0                0         0
000000007A754150    215      8      5 library cache 1131      0              0                0         0
000000007A7541F0    215      7      5 library cache 1191      0              0                0         1
000000007A754290    215      6      5 library cache 611      0              0                0         0
000000007A754330    215      5      5 library cache 1189      0              0                0         0
000000007A7543D0    215      4      5 library cache 1165      0              0                0         0
000000007A754470    215      3      5 library cache 861      0              0                0         0
000000007A754510    215      2      5 library cache 1328      0              0                0         1
000000007A7545B0    215      1      5 library cache 1199      1              0                0         0
29 rows selected.
--//29个latch.
--//library cache latch数量与cpu数量有关.选取大于cpu数量的最接近的质数.我当前cpu数量24(实际上2个cpu, 每个Core Count:
--//6,Core Enabled: 6,Thread Count: 12),这样显示的24个cpu,我自己还第一次注意这个细节.
--//我简单验证修改cpu_count=8,library cache latch的数量是11.修改cpu_count=12,library cache latch的数量是13.大家可以自行验证.
--//注意必须重启才生效.

SCOTT@test> show parameter cpu_count
NAME TYPE VALUE
--------- -------- -----
cpu_count integer 24

SELECT addr
        ,latch#
        ,child#
        ,level#
        ,name
        ,gets
        ,sleeps
        ,immediate_gets
        ,immediate_misses
        ,spin_gets
    FROM V$LATCH_CHILDREN
   WHERE name LIKE 'shared pool'
ORDER BY addr;

ADDR             LATCH# CHILD# LEVEL# NAME         GETS SLEEPS IMMEDIATE_GETS IMMEDIATE_MISSES SPIN_GETS
---------------- ------ ------ ------ ----------- ----- ------ -------------- ---------------- ---------
00000000600E7840    214      1      7 shared pool 43351      0              0                0       849
00000000600E78E0    214      2      7 shared pool     8      0              0                0         0
00000000600E7980    214      3      7 shared pool     8      0              0                0         0
00000000600E7A20    214      4      7 shared pool     8      0              0                0         0
00000000600E7AC0    214      5      7 shared pool     8      0              0                0         0
00000000600E7B60    214      6      7 shared pool     8      0              0                0         0
00000000600E7C00    214      7      7 shared pool     8      0              0                0         0
7 rows selected.
--//注意仅仅一个shared pool latch的gets很大,其它实际上启动后不会使用.
--//shared pool latch的数量与共享池内存大小,cpu数量有关.实际上取共享池内存大小/512M(对于11G是这样,早期版本有一些除256M,128M)
--//以及cpu数量/4的最小值.我设置sga才484M.仅仅1个shared pool latch.
--//另外latch实际上共享池内存的一片区域.你可以注意.相邻地址相减大小一样,相当于数组.比如:
--//0x600E7840=1611561024
--//0x600E78E0=1611561184
--//600E7980=1611561344
--//1611561184-1611561024 = 160
--//1611561344-1611561184 = 160

2.测试前准备:
--//打开3个会话窗口,将一些需要执行的sql语句执行多次,避免测试时硬解析时挂起.
--//desc dept.
--//select * from dept ;
--// @ &r/wait10g
--//Select * from dept where deptno=12;

3.测试1:
--//session 1:
SCOTT@test> @ &r/spid
SID SERIAL# PROCESS SERVER SPID PID P_SERIAL# C50
---------- ---------- ------------ --------- ------ ------- ---------- --------------------------------------------------
157 5 13448 DEDICATED 13449 17 3 alter system kill session '157,5' immediate;

--//session 2:
SYS@test> oradebug setmypid
Statement processed.

SYS@test> oradebug peek 0x00000000600E7840 4
[0600E7840, 0600E7844) = 00000000

SYS@test> oradebug poke 0x00000000600E7840 4 0x00000001
BEFORE: [0600E7840, 0600E7844) = 00000000
AFTER: [0600E7840, 0600E7844) = 00000001
--//给shared pool加上latch.
--//session 1:(第1次执行)
SCOTT@test> select * from dept where deptno=12;
--//注意seelect小写与前面不同,主要是执行一次硬解析,同时避开一些递归语句.

--//session 3:
SYS@test> @ &r/wait10g
P1RAW            P2RAW            P3RAW         P1         P2         P3        SID    SERIAL#       SEQ# EVENT              STATE   WAIT_TIME SECONDS_IN_WAIT
---------------- ---------------- ----- ---------- ---------- ---------- ---------- ---------- ---------- ------------------ ------- --------- ---------------
00000000600E7840 00000000000000D6 00    1611561024        214          0        157          5        114 latch: shared pool WAITING         0              60
00000000600E7840 00000000000000D6 00    1611561024        214          0        162          1       3266 latch: shared pool WAITING         0              66
00000000600E7840 00000000000000D6 00    1611561024        214          0        160          1         44 latch: shared pool WAITING         0              90
00000000600E7840 00000000000000D6 00    1611561024        214          0        159          1        625 latch: shared pool WAITING         0              69

--//session 2:
SYS@test> oradebug dump systemstate 266
Statement processed.

SYS@test> oradebug poke 0x00000000600E7840 4 0x00000000
BEFORE: [0600E7840, 0600E7844) = 000000FF
AFTER: [0600E7840, 0600E7844) = 00000000

--//这样挂起会话中语句执行成功!!
SYS@test> @ &r/wait10g
no rows selected

--//说明硬解析的sql语句需要获取shared pool.

4.检查转储文件内容:
PROCESS 17:
----------------------------------------
SO: 0x7ce63c10, type: 2, owner: (nil), flag: INIT/-/-/0x00
(process) Oracle pid=17, calls cur/top: 0x7cfa89b0/0x7cfa89b0, flag: (0) -
            int error: 0, call error: 0, sess error: 0, txn error 0
(post info) last post received: 0 0 0
              last post received-location: No post
              last process to post me: none
              last post sent: 0 0 33
              last post sent-location: ksrpublish
              last process posted by me: 7ce604b8 1 22
    (latch info) wait_event=0 bits=20
        Location from where call was made: kghalo:
      waiting for 600e7840 Child shared pool level=7 child#=1
        Location from where latch is held: kghupr1: Chunk Header
        Context saved from call: 1986860088
        state=busy, wlstate=free
          waiters [orapid (seconds since: put on list, posted, alive check)]:
           12 (197, 1552958147, 2)
           13 (176, 1552958147, 2)
           10 (173, 1552958147, 5)
           17 (167, 1552958147, 167)
           waiter count=4
          gotten 75402 times wait, failed first 851 sleeps 0
          gotten 0 times nowait, failed: 0
      on wait list for 600e7840
      holding    (efd=4) 7a7537f0 Child library cache level=5 child#=23
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        Location from where latch is held: kglhdgn: child:: latch
        Context saved from call: 6
        state=busy, wlstate=free
    Process Group: DEFAULT, pseudo proc: 0x7cea6ac8
    O/S info: user: oracle, term: pts/3, ospid: 13449
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    OSD pid info: Unix process pid: 13449, image: oracle@xxxxdg (TNS V1-V3)
    Short stack dump:

--//ospid: 13449,注意看下划线内容,指示它还持有library cache level=5 child#=23.对于child#=23.也就是第一次硬解析还需要持有
--//library cache latch,是否相同的sql语句每次解析都需要持有library cache latch,而且child#都是一样的呢?
--//可以推测应该一样,假设2条相同的sql语句需要硬解析,如果持有library cache latch的child#不一样,这样有可能出现2个光标在共
--//享池?所以推测下次执行相同语句使用的library cache latch的child#应该与前一次相同.

5.继续测试:
--//session 2:
SYS@test> oradebug peek 0x000000007A7537F0 4
[07A7537F0, 07A7537F4) = 00000000

SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000001
BEFORE: [07A7537F0, 07A7537F4) = 00000000
AFTER: [07A7537F0, 07A7537F4) = 00000001
--//给对应library cache 加latch,child#=23.

--//session 1:(第2次执行)
SCOTT@test> select * from dept where deptno=12;
--//再次挂起!

--//这就是软解析,需要持有library cache ltach.P1raw=000000007A7537F0也可以对上.
--//注:另外补充说明一下第2,3次执行软解析也需要1次shared pool latch.
--//session 2:
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000000
BEFORE: [07A7537F0, 07A7537F4) = 000000FF
AFTER: [07A7537F0, 07A7537F4) = 00000000

--//session 1:(第2次执行)
SCOTT@test> select * from dept where deptno=12;
no rows selected
--//执行成功!!继续:

--//session 2:
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000001
BEFORE: [07A7537F0, 07A7537F4) = 00000000
AFTER: [07A7537F0, 07A7537F4) = 00000001

--//session 1:(第3次执行)
SCOTT@test> select * from dept where deptno=12;
--//再次挂起!

--//session 2:
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000000
BEFORE: [07A7537F0, 07A7537F4) = 000000FF
AFTER: [07A7537F0, 07A7537F4) = 00000000

--//session 1:
SCOTT@test> select * from dept where deptno=12;
no rows selected

6.测试2:
--//当前已经执行select * from dept where deptno=12;3次,如果session_cached_cursors非0的情况下,该光标已经被缓存,也就是相同
--//会话的情况下次执行,就是软软解析,不需要在持有library cache ltach.
--//session 3:
SYS@test> select * from v$open_cursor where SID=157;
SADDR                   SID USER_NAME ADDRESS          HASH_VALUE SQL_ID        SQL_TEXT
---------------- ---------- --------- ---------------- ---------- ------------- ----------------------------------
000000007CF80200        157 SCOTT     000000007683E920   30432287 2f476y80x0r0z select * from dept where deptno=12
000000007CF80200        157 SCOTT     000000007C9C1D68 3393152264 4gd6b1r53yt88 table_1_ff_14f_0_0_0

--//session 2:
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000001
BEFORE: [07A7537F0, 07A7537F4) = 00000000
AFTER: [07A7537F0, 07A7537F4) = 00000001

SYS@test> oradebug poke 0x00000000600E7840 4 0x00000001
BEFORE: [0600E7840, 0600E7844) = 00000000
AFTER: [0600E7840, 0600E7844) = 00000001

--//session 1:
SCOTT@test> select * from dept where deptno=12;
no rows selected

--//执行成功!!也就是当光标缓存后,sql语句执行不需要在持有对应的library cache latch,oracle通过这样的方式(软解析,软软解析)减少latch的使用.

7.测试3:
--//如果打开新的会话窗口,再次执行select * from dept where deptno=12;,对于新会话并没有缓存该光标,第1次执行应该也是软解析.
--//通过测试说明问题.
--//session 2:
SYS@test> oradebug poke 0x00000000600E7840 4 0x00000000
BEFORE: [0600E7840, 0600E7844) = 000000FF
AFTER: [0600E7840, 0600E7844) = 00000000
--//首先取消shared pool latch.注意library cache latch还在.child#=23.
--//注:另外补充说明一下第2,3次执行软解析也需要1次shared pool latch.估计这样不允许这时建立新的子光标.

--//session 4:
SCOTT@test> @ &r/spid

SID SERIAL# PROCESS SERVER SPID PID P_SERIAL# C50
---------- ---------- ------------ --------- ------ ------- ---------- --------------------------------------------------
141 45 13648 DEDICATED 13649 25 3 alter system kill session '141,45' immediate;

SCOTT@test> select * from dept where deptno=12;
--//再次挂起!!

--//sid=141,正是新打开的会话sid=141.

--//session 2:取消library cache latch.
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000000
BEFORE: [07A7537F0, 07A7537F4) = 000000FF
AFTER: [07A7537F0, 07A7537F4) = 00000000

--//session 4:
SCOTT@test> select * from dept where deptno=12;
no rows selected
--//正常执行!!也就是新打开会话即使其它会话已经缓存光标,第1次执行也是软解析.
--//如果会话再次执行,应该该光标缓存,就不再需要对应的library cache latch.

--//session 2:
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000001
BEFORE: [07A7537F0, 07A7537F4) = 00000000
AFTER: [07A7537F0, 07A7537F4) = 00000001

--//session 4:
SCOTT@test> select * from dept where deptno=12;
no rows selected
--//ok,执行成功!!也就是这次是软软解析.

8.测试3:
--//如果会话改变参数,这样会产生新的子光标,看看这样的情况会持有那些latch.
--//session 1:
SCOTT@test> show parameter optimizer_index_caching
NAME TYPE VALUE
----------------------- ------- -----
optimizer_index_caching integer 0

SCOTT@test> alter session set optimizer_index_caching=1;
Session altered.
--//修改参数optimizer_index_caching,这样如果执行select * from dept where deptno=12;会产生新的子光标.

--//session 2:
SYS@test> oradebug poke 0x00000000600E7840 4 0x00000001
BEFORE: [0600E7840, 0600E7844) = 00000000
AFTER: [0600E7840, 0600E7844) = 00000001
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000001
BEFORE: [07A7537F0, 07A7537F4) = 00000000
AFTER: [07A7537F0, 07A7537F4) = 00000001

--//session 1:(修改参数后第1次执行)
SCOTT@test> select * from dept where deptno=12;

--//挂起!!
--//session 3:
SYS@test> @ &r/wait10g
P1RAW            P2RAW            P3RAW         P1         P2         P3        SID    SERIAL#       SEQ# EVENT                STATE   WAIT_TIME SECONDS_IN_WAIT
---------------- ---------------- ----- ---------- ---------- ---------- ---------- ---------- ---------- -------------------- ------- --------- ---------------
000000007A7537F0 00000000000000D7 00    2054502384        215          0        157          5        172 latch: library cache WAITING         0              21
00000000600E7840 00000000000000D6 00    1611561024        214          0        160          1         59 latch: shared pool   WAITING         0              48
00000000600E7840 00000000000000D6 00    1611561024        214          0        159          1        799 latch: shared pool   WAITING         0              45

--//注意仅仅看sid=157行,还需要library cache latch,因为父光标已经存在.

--//session 2:取消对应library cache latch.
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000000
BEFORE: [07A7537F0, 07A7537F4) = 000000FF
AFTER: [07A7537F0, 07A7537F4) = 00000000

--//session 1:检查发现会话一样挂起!!
SCOTT@test> select * from dept where deptno=12;

--//session 3:
SYS@test> @ &r/wait10g
P1RAW            P2RAW            P3RAW         P1         P2         P3        SID    SERIAL#       SEQ# EVENT                 STATE   WAIT_TIME SECONDS_IN_WAIT
---------------- ---------------- ----- ---------- ---------- ---------- ---------- ---------- ---------- --------------------- ------- --------- ---------------
00000000600E7840 00000000000000D6 00    1611561024        214          0        157          5        173 latch: shared pool    WAITING         0              51
00000000600E7840 00000000000000D6 00    1611561024        214          0        160          1         59 latch: shared pool    WAITING         0             192
00000000600E7840 00000000000000D6 00    1611561024        214          0        159          1        799 latch: shared pool    WAITING         0             189
--//可以发现现在等待事件变成了latch: shared pool(sid=157).可以发现要因为参数的变化,要生产新的子光标,需要持有shared pool
--//latch 从共享池内存建立子光标.

--//session 2:取消shared pool latch.
SYS@test> oradebug poke 0x00000000600E7840 4 0x00000000
BEFORE: [0600E7840, 0600E7844) = 000000FF
AFTER: [0600E7840, 0600E7844) = 00000000

--//session 1:
SCOTT@test> select * from dept where deptno=12;
no rows selected
--//执行成功!!继续测试:

--//session 2: 再次设置latch.
SYS@test> oradebug poke 0x00000000600E7840 4 0x00000001
BEFORE: [0600E7840, 0600E7844) = 00000000
AFTER: [0600E7840, 0600E7844) = 00000001
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000001
BEFORE: [07A7537F0, 07A7537F4) = 00000000
AFTER: [07A7537F0, 07A7537F4) = 00000001

--//session 1:(修改参数后第2次执行)
SCOTT@test> select * from dept where deptno=12;

--//挂起!!

--//需要对应library cache latch.
--//session 2: 取消library cache latch.
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000000
BEFORE: [07A7537F0, 07A7537F4) = 000000FF
AFTER: [07A7537F0, 07A7537F4) = 00000000

--//session 1:
SCOTT@test> select * from dept where deptno=12;

--//还是挂起!!看看等待事件.

SYS@test> @ &r/wait10g
P1RAW P2RAW P3RAW P1 P2 P3 SID SERIAL# SEQ# EVENT STATE WAIT_TIME SECONDS_IN_WAIT
---------------- ---------------- ----- ---------- ---------- ---------- ---------- ---------- ---------- --------------------- ------- --------- ---------------
00000000600E7840 00000000000000D6 00 1611561024 214 0 157 5 177 latch: shared pool WAITING 0 12

--//奇怪还是需要持有shared pool latch.为什么??
--//session 2: 取消shared pool latch.
SYS@test> oradebug poke 0x00000000600E7840 4 0x00000000
BEFORE: [0600E7840, 0600E7844) = 000000FF
AFTER: [0600E7840, 0600E7844) = 00000000

--//session 1:(修改参数后第2次执行)
SCOTT@test> select * from dept where deptno=12;
no rows selected

--//不理解为什么第2次执行(在修改参数optimizer_index_caching后)还需要持有shared pool latch...
--//再来看看:
--//session 2:
SYS@test> oradebug poke 0x00000000600E7840 4 0x00000001
BEFORE: [0600E7840, 0600E7844) = 00000000
AFTER: [0600E7840, 0600E7844) = 00000001
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000001
BEFORE: [07A7537F0, 07A7537F4) = 00000000
AFTER: [07A7537F0, 07A7537F4) = 00000001

--//session 1:(修改参数后第3次执行)
SCOTT@test> select * from dept where deptno=12;

--//session 2:取消library cache latch.
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000000
BEFORE: [07A7537F0, 07A7537F4) = 000000FF
AFTER: [07A7537F0, 07A7537F4) = 00000000

--//session 1:
SCOTT@test> select * from dept where deptno=12;

--//session 2: 取消shared pool latch.
SYS@test> oradebug poke 0x00000000600E7840 4 0x00000000
BEFORE: [0600E7840, 0600E7844) = 000000FF
AFTER: [0600E7840, 0600E7844) = 00000000

--//session 1:
SCOTT@test> select * from dept where deptno=12;
no rows selected

--//执行成功!!

--//session 2:再来看看:
SYS@test> oradebug poke 0x00000000600E7840 4 0x00000001
BEFORE: [0600E7840, 0600E7844) = 00000000
AFTER: [0600E7840, 0600E7844) = 00000001
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000001
BEFORE: [07A7537F0, 07A7537F4) = 00000000
AFTER: [07A7537F0, 07A7537F4) = 00000001

--//session 1:
SCOTT@test> select * from dept where deptno=12;
no rows selected

--//可以发现同一会话如果产生新的子光标,第1,2,3次执行都需要library cache latch和shared pool latch.第4次执行光标已经cache,不再需要持有对应latch.

9.测试4:
--//再回到session 4,参数没有变化optimizer_index_caching变化.看看现在的情况(该会话已经执行)select * from dept where deptno=12; 2次:
--//session 2:
SYS@test> oradebug poke 0x00000000600E7840 4 0x00000001
BEFORE: [0600E7840, 0600E7844) = 00000001
AFTER: [0600E7840, 0600E7844) = 00000001
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000001
BEFORE: [07A7537F0, 07A7537F4) = 000000FF
AFTER: [07A7537F0, 07A7537F4) = 00000001

--//session 4:
SCOTT@test> select * from dept where deptno=12;
no rows selected
--//软软解析依旧有效.

SYS@test> select * from v$open_cursor where SID in (157,141) and sql_text like '%deptno%';
SADDR                   SID USER_NAME ADDRESS          HASH_VALUE SQL_ID        SQL_TEXT
---------------- ---------- --------- ---------------- ---------- ------------- ----------------------------------
000000007CF80200        157 SCOTT     000000007683E920   30432287 2f476y80x0r0z select * from dept where deptno=12
000000007CF6AB80        141 SCOTT     000000007683E920   30432287 2f476y80x0r0z select * from dept where deptno=12

10.测试5:
--//修改session 4参数:
--//session 4:
SCOTT@test> alter session set optimizer_index_caching=1;
Session altered.

--//session 4:
SCOTT@test> select * from dept where deptno=12;

--//挂起!!
--//session 2:
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000000
BEFORE: [07A7537F0, 07A7537F4) = 00000001
AFTER: [07A7537F0, 07A7537F4) = 00000000

--//session 4:
SCOTT@test> select * from dept where deptno=12;
no rows selected

--//session 2:
SYS@test> oradebug poke 0x000000007A7537F0 4 0x00000001
BEFORE: [07A7537F0, 07A7537F4) = 00000000
AFTER: [07A7537F0, 07A7537F4) = 00000001
SYS@test> oradebug poke 0x00000000600E7840 4 0x00000001
BEFORE: [0600E7840, 0600E7844) = 00000000
AFTER: [0600E7840, 0600E7844) = 00000001

--//session 4:
SCOTT@test> select * from dept where deptno=12;
no rows selected
--//软软解析!!

11.总结:
--//不好写.11g已经不存在这个shared pool latch,改为mutexes.所以测试在10g下进行!!
--//硬解析先要library cache latch,估计查询该光标是否在共享池存在,如果不存在,持有shared pool latch,使用共享池内存建立父子光标.
--//再持有library cache latch.关于这点从前面的转储可以发现.但是等待事件先出现的是shared pool latch,然后才是library cache latch.

--//软解析的情况要正常需要持有library cache latch以及shared pool latch.
--//软软解析不需要持有对应的library cache latch和shared pool latch.
--//即使光标缓存的情况下,新建立的会话第1次执行也需要持有library cache latch.shared pool latch.
--//如果某种原因产生子光标的情况下,第1,2,3次执行都需要library cache latch和shared pool latch.第4次执行光标已经cache,不再需要持有对应
--//library cache latch和shared pool latch.