首先先分析一下Alter Index Online的整个过程，只是一个分析思路而且这个行为是很有代表性的，其他的操作都是大同小异。
1 Session 1 中建表
SQL> create table xxd_t as
2 select rownum id,dbms_random.string('l',20) user_name
3 from dual
4 connect by level <= 5e6;
Table created.
2 Session 1 中建索引
SQL> create index xxd_t_pk on xxd_t (id);
Index Created
3 Session 2中Update表制造阻塞
SQL> update xxd_t set user_name = 'test 1' where rownum <= 1;
4 Session 3中看到session 1被堵塞在Table Share Lock的Request模式时进行下一步.
SQL> select /*+ rule*/a.* from v$lock a,v$session b where a.sid = b.sid and b.username = user;
ADDR KADDR SID TY ID1 ID2 LMODE REQUEST CTIME BLOCK
-------- -------- ---------- -- ---------- ---------- ---------- ---------- ---------- ----------
405C3F90 405C3FA8 159 TM 61291 0 4 0 51 1
405C40E8 405C4100 143 TM 61291 0 0 3 4 0
405C403C 405C4054 159 TM 18 0 3 0 51 0
41BB9D58 41BB9D6C 159 DL 61291 0 3 0 51 0
41BB9CA0 41BB9CB4 159 DL 61291 0 3 0 51 0
405C34E8 405C3530 159 TS 0 4285569 6 0 30 0
40619440 4061955C 159 TX 524295 3868 6 0 51 0
7 rows selected
4. 在Session 1 (运行index rebuild online的进程)运行如下语句.
alter session set events '10704 trace name context forever,level 10';
alter session set events '10046 trace name context forever,level 12';
alter index xxd_t_pk rebuild online;
5. 在Session 2 提交前一个事务,使得rebuild online 过程继续,并运行一个需要大量index字段的更新操作.构造需要rebuild online在获取下一次Table Share lock之前需要Merge的数据.
commit;
update xxd_t set id = 5000000 + id where rownum <= 3e5;
commit;
6. 在Session 2上运行一个类似于第二步的简单更新,意在阻塞Rebuild Online获取Table Share Lock.
update xxd_t set user_name = 'test 1' where rownum <= 1;
7. 在Session 3中观察,当观察到session 1被堵塞在Table Share Lock的Request模式时,执行下面的语句后进行下一步.清空Buffer Cache以观察Session 1在获取Table Share Lock后执行的操作.
alter session set events = 'immediate trace name flush_cache';
8. 提交Session 2的事务, 等待Rebuild Online结束..
Commit;
9. Trace文件中整个测试过程相关内容，随后进行分析
SQL> select object_id from dba_objects where object_name='XXD_T';
61291
SQL> select to_char('61291','xxxxxx') from dual;
TO_CHAR
-------
ef6b
SQL> select object_id from dba_objects where object_name='XXD_T_PK';
OBJECT_ID
----------
61292
SQL> select to_char('61292','xxxxxx') from dual;
TO_CHAR
-------
ef6c
node1*orcl-/u01/app/oracle/admin/orcl/udump >more orcl_ora_18245.trc | grep -n 'ef6b'
84:ksqgtl *** DL-0000ef6b-00000000 mode=3 flags=0x11 timeout=0 ***
98:ksqgtl *** DL-0000ef6b-00000000 mode=3 flags=0x11 timeout=0 ***
111:ksqcmi: DL,ef6b,0 mode=3 timeout=0
120:ksqgtl *** TM-0000ef6b-00000000 mode=2 flags=0x401 timeout=21474836 ***
133:ksqcmi: TM,ef6b,0 mode=2 timeout=21474836
2949:ksqcnv: TM-0000ef6b,00000000 mode=4 timeout=21474836
2951:ksqcmi: TM,ef6b,0 mode=4 timeout=21474836
2964:ksqcnv: TM-0000ef6b,00000000 mode=2 timeout=21474836
2966:ksqcmi: TM,ef6b,0 mode=2 timeout=21474836
40346:ksqcnv: TM-0000ef6b,00000000 mode=4 timeout=21474836
40348:ksqcmi: TM,ef6b,0 mode=4 timeout=21474836
80569:ksqrcl: DL,ef6b,0
80572:ksqrcl: DL,ef6b,0
82637:ksqrcl: TM,ef6b,0
展开分析一下
84*** 2007-02-14 07:07:20.517
ksqgtl *** DL-0000ef6b-00000000 mode=3 flags=0x11 timeout=0 ***
ksqgtl: xcb=0x0x40619e88, ktcdix=2147483647, topxcb=0x0x40619e88
ktcipt(topxcb)=0x0
98*** 2007-02-14 07:07:20.517
ksqgtl *** DL-0000ef6b-00000000 mode=3 flags=0x11 timeout=0 ***
ksqgtl: xcb=0x0x40619e88, ktcdix=2147483647, topxcb=0x0x40619e88
ktcipt(topxcb)=0x0
111*** 2007-02-14 07:07:20.517
ksqcmi: DL,ef6b,0 mode=3 timeout=0
ksqcmi: returns 0
ksqgtl: RETURNS 0
120*** 2007-02-14 07:07:20.518 首先获得表的行级共享锁mode=2 SS(Row-S)
ksqgtl *** TM-0000ef6b-00000000 mode=2 flags=0x401 timeout=21474836 ***
ksqgtl: xcb=0x0x40619e88, ktcdix=2147483647, topxcb=0x0x40619e88
ktcipt(topxcb)=0x0
133 *** 2007-02-14 07:07:20.518
ksqcmi: TM,ef6b,0 mode=2 timeout=21474836
ksqcmi: returns 0
ksqgtl: RETURNS 0
284 PARSING IN CURSOR #5 len=158 dep=1 uid=0 oct=1 lid=0 tim=1267225039604101 hv=4036323617 ad='3c8d8428'
create table "SYS"."SYS_JOURNAL_61292" (C0 NUMBER, opcode char(1), partno number, rid rowid, primary key( C0 , rid )) organizatio\
n index TABLESPACE "SYSTEM"
END OF STMT
2949 *** 2007-02-14 07:07:20.665 Alter Index Online发生后，此时请求表的mode=4 Share锁
ksqcnv: TM-0000ef6b,00000000 mode=4 timeout=21474836
2951*** 2007-02-14 07:07:20.665
ksqcmi: TM,ef6b,0 mode=4 timeout=21474836
WAIT #4: nam='enq: TM - contention' ela= 3 name|mode=1414332420 object #=61291 table/partition=0 obj#=61291 tim=1267225039712868
WAIT #4: nam='enq: TM - contention' ela= 2931299 name|mode=1414332420 object #=61291 table/partition=0 obj#=61291 tim=1267225042648\
585
WAIT #4: nam='enq: TM - contention' ela= 2931312 name|mode=1414332420 object #=61291 table/partition=0 obj#=61291 tim=1267225045580\
002
WAIT #4: nam='enq: TM - contention' ela= 2930320 name|mode=1414332420 object #=61291 table/partition=0 obj#=61291 tim=1267225048510\
425
*** 2011-02-14 07:07:32.676
WAIT #4: nam='enq: TM - contention' ela= 2931245 name|mode=1414332420 object #=61291 table/partition=0 obj#=61291 tim=1267225051441\
775
WAIT #4: nam='enq: TM - contention' ela= 2931251 name|mode=1414332420 object #=61291 table/partition=0 obj#=61291 tim=1267225054373\
156
WAIT #4: nam='enq: TM - contention' ela= 2931269 name|mode=1414332420 object #=61291 table/partition=0 obj#=61291 tim=1267225057304\
541
WAIT #4: nam='enq: TM - contention' ela= 2463661 name|mode=1414332420 object #=61291 table/partition=0 obj#=61291 tim=1267225059768\
303
ksqcmi: returns 0
ksqcnv: RETURNS 0
2964 *** 2007-02-14 07:07:41.202
ksqcnv: TM-0000ef6b,00000000 mode=2 timeout=21474836
2966 *** 2007-02-14 07:07:41.202
ksqcmi: TM,ef6b,0 mode=2 timeout=21474836
ksqcmi: returns 0
ksqcnv: RETURNS 0
WAIT #4: nam='direct path read temp' ela= 7 file number=201 first dba=1189 block cnt=1 obj#=61291 tim=1267225169849896
WAIT #4: nam='direct path read temp' ela= 3 file number=201 first dba=1190 block cnt=1 obj#=61291 tim=1267225169851138
WAIT #4: nam='direct path write' ela= 1 file number=1 first dba=138185 block cnt=7 obj#=61291 tim=1267225169874164
WAIT #4: nam='direct path write' ela= 23 file number=1 first dba=138185 block cnt=7 obj#=61291 tim=1267225169874186
WAIT #4: nam='direct path write' ela= 0 file number=1 first dba=138192 block cnt=7 obj#=61291 tim=1267225169874645
WAIT #4: nam='direct path write' ela= 16 file number=1 first dba=138192 block cnt=7 obj#=61291 tim=1267225169874660
*** 2007-02-14 07:09:44.308
WAIT #4: nam='buffer busy waits' ela= 435 file#=2 block#=8768 class#=30 obj#=0 tim=1267225179988859
WAIT #4: nam='buffer busy waits' ela= 97288 file#=2 block#=8768 class#=30 obj#=0 tim=1267225180086260
*** 2007-02-14 07:09:44.520
ksqcnv: TM-0000ef6b,00000000 mode=4 timeout=21474836
*** 2007-02-14 07:09:44.520
ksqcmi: TM,ef6b,0 mode=4 timeout=21474836
WAIT #4: nam='enq: TM - contention' ela= 8 name|mode=1414332420 object #=61291 table/partition=0 obj#=0 tim=1267225180195810
WAIT #4: nam='enq: TM - contention' ela= 2930231 name|mode=1414332420 object #=61291 table/partition=0 obj#=0 tim=1267225183126073
WAIT #4: nam='enq: TM - contention' ela= 2930254 name|mode=1414332420 object #=61291 table/partition=0 obj#=0 tim=1267225186056414
WAIT #4: nam='enq: TM - contention' ela= 2930360 name|mode=1414332420 object #=61291 table/partition=0 obj#=0 tim=1267225188986878
*** 2007-02-14 07:09:56.524
……………………….
*** 2007-02-14 07:13:53.253
ksqrcl: DL,ef6b,0
ksqrcl: returns 0
*** 2007-02-14 07:13:53.254
ksqrcl: DL,ef6b,0
ksqrcl: returns 0
XCTEND rlbk=0, rd_only=0
=====================
PARSING IN CURSOR #4 len=35 dep=0 uid=0 oct=9 lid=0 tim=1267225039568125 hv=3160315829 ad='3c8d229c'
alter index xxd_t_pk rebuild online
END OF STMT
PARSE #4:c=14997,e=52795,p=0,cr=10,cu=0,mis=1,r=0,dep=0,og=1,tim=1267225039568120
*** 2007-02-14 07:07:20.518
ksqgtl *** TM-0000ef6b-00000000 mode=2 flags=0x401 timeout=21474836 ***
ksqgtl: xcb=0x0x40619e88, ktcdix=2147483647, topxcb=0x0x40619e88
ktcipt(topxcb)=0x0
*** 2007-02-14 07:07:20.518
ksucti: init session DID from txn DID: 0001-0012-000003B2
ksqgtl:
ksqlkdid: 0001-0012-000003B2
*** 2007-02-14 07:07:20.518
*** ksudidTrace: ksqgtl
ktcmydid(): 0001-0012-000003B2
ksusesdi: 0000-0000-00000000
ksusetxn: 0001-0012-000003B2
*** 2007-02-14 07:07:20.518
ksqcmi: TM,ef6b,0 mode=2 timeout=21474836
ksqcmi: returns 0
ksqgtl: RETURNS 0
BINDS #4:
=====================
*** 2007-02-14 07:07:20.521
ksqgtl *** TX-00060025-00000f28 mode=6 flags=0x401 timeout=0 ***
ksqgtl: xcb=0x0x4060e3c8, ktcdix=2147483647, topxcb=0x0x40619e88
ktcipt(topxcb)=0x0
*** 2007-02-14 07:07:20.521
ksucti: init session DID from txn DID: 0001-0012-000003B2
ksqgtl:
ksqlkdid: 0001-0012-000003B2
*** 2011-02-14 07:07:20.521
*** ksudidTrace: ksqgtl
ktcmydid(): 0001-0012-000003B2
ksusesdi: 0000-0000-00000000
ksusetxn: 0001-0012-000003B2
ksqgtl: RETURNS 0
EXEC #2:c=2000,e=1663,p=0,cr=2,cu=2,mis=0,r=1,dep=1,og=4,tim=1267225039571657
STAT #2 id=1 cnt=0 pid=0 pos=1 obj=0 op='UPDATE OBJ$ (cr=2 pr=0 pw=0 time=465 us)'
STAT #2 id=2 cnt=1 pid=1 pos=1 obj=37 op='INDEX RANGE SCAN I_OBJ2 (cr=2 pr=0 pw=0 time=114 us)'
*** 2007-02-14 07:07:20.521
ksqrcl: TX,60025,f28
ksqrcl: returns 0
*** 2007-02-14 07:07:20.521
ksqrcl: TM,12,0
ksqrcl: returns 0
*** 2007-02-14 07:07:20.521
ksqgtl *** TT-00000000-00000000 mode=4 flags=0x411 timeout=21474836 ***
ksqgtl: xcb=0x0x4061a3a4, ktcdix=2147483647, topxcb=0x0x40619e88
ktcipt(topxcb)=0x0
*** 2007-02-14 07:07:20.521
ksucti: init txn DID from session DID 0001-0012-000003B2
ksqgtl:
ksqlkdid: 0001-0012-000003B2
*** 2011-02-14 07:07:20.521
*** ksudidTrace: ksqgtl
ktcmydid(): 0001-0012-000003B2
ksusesdi: 0000-0000-00000000
ksusetxn: 0001-0012-000003B2
ksqgtl: RETURNS 0
=====================
总结一下Alter Index Online的过程
1. 创建日志表
2. 首先获得表的行级共享锁mode=2 SS(Row-S)
3. 对XXD_T_PK申请了 Mode=6的锁，这时禁止对碎银进行DDL操作
4. Alter Index Online发生后，此时请求表的mode=4 Share锁
5. 这时索引排序，伴随着异步IO kfk: async disk IO，并行写入Temp(direct path write temp)，读Temp（direct path read temp），创建索引，写入索引段，索引段扩展(enq: HW - contention)，最后这些结束后，写入索引(direct path write)，更新数据字典(update ind$)，跟新索引段信息(update seg$)
再总结下其他操作的，这里例子就不给出了
Truncate Table T;
mode=6 exclusive (X)
Create index
Create Index idx_test on T;
mode=4的会阻塞mode=3的请求，所以create index会阻塞dml操作
对比create index和create index online的区别
Alter Index Online 加的是DDL锁，没有Online关键词加的是DDL和DML锁
create index online 加的是mode=2 (RS)，所以不会阻塞mode=3(RX)的RX锁，所以不会阻挡DML。但是会阻塞大部分的DDL加在表上的X类型的TM锁
create index：加的是mode=4(S)的TM锁，会阻塞DML语句加在表上面的类型为RX的TM锁。这时，表锁都被阻塞，那么行锁更是不可能加载的。
Related Post：
N/A
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