create index or add online区别

一直以来经验告诉我们，创建索引需要加online？ 为啥？

总结：

1.创建索引不加online, 表有dml操作未结束事务，创建索引失败；

2.创建索引不加online,在建索引的过程中，业务对表有dml操作，业务将被阻塞tm锁无法申请这个资源【大表建索引影响更大】；

3.创建索引使用online,表有dml操作未结束事务，创建索引无法成功,会话处于tx行锁争用，表不存在相关事务后，建索引自动成功；

4.创建索引使用online,在建索引的过程中,业务对表dml操作,业务dml操作将顺利执行，直到表无任何事务后，索引建成功；

5.创建索引使用Online,在建索引的过程中，使用ctrl+c或其他原因导致索引创建失败，数据字典将残留对象需要调用存储过程进行清理。

创建索引测试锁



1.对表dml操作,事务未结束
session1
SQL> delete d1 where rownum=1;
1 row deleted.
session2
SQL> create index c_id on d1(object_id);
create index c_id on d1(object_id)
                     *
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired

分析create index 锁
session2
SQL> alter session set ddl_lock_timeout=600;
SQL> create index c_id on d1(object_id);
hang 

session3
SQL> delete d1 where object_id>9000;
hang

SQL>  select INST_ID,sid,serial#,USERNAME,STATUS,MACHINE,SQL_ID,EVENT,(sysdate-LOGON_TIME)*86400 as "s",LAST_CALL_ET from gv$session where status='ACTIVE' and username is not null;
INST_ID    SID SERIAL# USERNAME             STATUS     MACHINE              SQL_ID               EVENT                                s LAST_CALL_ET
------- ------ ------- -------------------- ---------- -------------------- -------------------- ------------------------------ ------- ------------
      2      1    4711 SYS                  ACTIVE     d2                   2ymxxw3mapxd9        PX Deq: Execution Msg                0            0
      2     19   12269 SYS                  ACTIVE     d2                   2k397xzajspfu        enq: TM - contention               290           54
      2     30     424 SYS                  ACTIVE     d2                   7ddkb1x0zhpp5        enq: TM - contention                49           22
      2     43   42485 SYS                  ACTIVE     d2                   2ymxxw3mapxd9        PX Deq: Execute Reply               16            0
      2    402    5717 SYS                  ACTIVE     d2                                        OFS idle                          1130         1130
      1      3   30306 SYS                  ACTIVE     d2                   2ymxxw3mapxd9        PX Deq: Execution Msg                0            0
      1    403   22969 SYS                  ACTIVE     d1                                        OFS idle                          1494         1495
7 rows selected.
SQL> select object_id from dba_objects where owner='SYS' and object_name='D1';
 OBJECT_ID
----------
     73029

SQL> select inst_id,sid,type,id1,lmode,request,block from gv$lock where type in('TX','TM');
INST_ID    SID TY        ID1      LMODE    REQUEST      BLOCK
------- ------ -- ---------- ---------- ---------- ----------
        31 TM      73029          3          0          1
        31 TX     524300          6          0          0
        29 TM      73029          0          3          0
        28 TM      73029          0          4          0
Session 1
SQL> commit;
Session 2
OK  index create complete
Session 3
OK  delete complete.  

SQL>  select INST_ID,sid,serial#,USERNAME,SQL_ID,EVENT,(sysdate-LOGON_TIME)*86400 as "s",LAST_CALL_ET,BLOCKING_INSTANCE,BLOCKING_SESSION,FINAL_BLOCKING_INSTANCE,FINAL_BLOCKING_SESSION from gv$session where 
 SID IN(19,30) and inst_id=2;
INST_ID    SID SERIAL# USERN SQL_ID               EVENT                      s LAST_CALL_ET BLOCKING_INSTANCE BLOCKING_SESSION FINAL_BLOCKING_INSTANCE FINAL_BLOCKING_SESSION
------- ------ ------- ----- -------------------- -------------------- ------- ------------ ----------------- ---------------- ----------------------- ----------------------
      2     19   12269 SYS   2k397xzajspfu        enq: TM - contention     954           91                 1               17                       1                     17
      2     30     424 SYS   7ddkb1x0zhpp5        enq: TM - contention     713           88
SQL>SELECT SQL_TEXT FROM V$SQL WHERE SQL_ID='7ddkb1x0zhpp5';
SQL_TEXT
----------------------------------------
delete d1 where object_id>9000
SQL>SELECT SQL_TEXT FROM V$SQL WHERE SQL_ID='2k397xzajspfu';
这点很奇怪,说明事物未结束,索引将无法创建成功，并且会阻塞后续的dml操作。 但是申请表锁的级别无法得到。
上述出现锁是由于对创建索引的session  ddl锁申请资源参数进行调整,默认是0，并不会出现创建索引不成功，反而阻塞后续业务DML操作。
SQL> alter session set ddl_lock_timeout=0;

2.使用online参数
session1
SQL> delete d1 where rownum=1;
1 row deleted.
session2
SQL> create index c_id on d1(object_id) online;

session3
SQL> delete d1 where object_id>9000;
63382 rows deleted. --可以发现使用online创建索引，但是在索引创建session并未结束时, dml不影响！！！

SQL>  select INST_ID,sid,serial#,USERNAME,STATUS,MACHINE,SQL_ID,EVENT,(sysdate-LOGON_TIME)*86400 as "s",LAST_CALL_ET from gv$session where status='ACTIVE' and username is not null;
INST_ID    SID SERIAL# USERNAME             STATUS     MACHINE              SQL_ID               EVENT                                s LAST_CALL_ET
------- ------ ------- -------------------- ---------- -------------------- -------------------- ------------------------------ ------- ------------
      2     19   12269 SYS                  ACTIVE     d2                   0g5b4q13v6gp4        enq: TX - row lock contention     1264           67
SQL>SELECT SQL_TEXT FROM V$SQL WHERE SQL_ID='0g5b4q13v6gp4';
SQL_TEXT
-----------------------------------------
create index c_id on d1(object_id) online
SQL>  select INST_ID,sid,serial#,USERNAME,SQL_ID,EVENT,(sysdate-LOGON_TIME)*86400 as "s",LAST_CALL_ET,BLOCKING_INSTANCE,BLOCKING_SESSION,FINAL_BLOCKING_INSTANCE,FINAL_BLOCKING_SESSION from gv$session where 
 SID IN(19) and inst_id=2;
INST_ID    SID SERIAL# USERNAME SQL_ID          EVENT                                s LAST_CALL_ET BLOCKING_INSTANCE BLOCKING_SESSION FINAL_BLOCKING_INSTANCE FINAL_BLOCKING_SESSION
------- ------ ------- -------- --------------- ------------------------------ ------- ------------ ----------------- ---------------- ----------------------- ----------------------
      2     19   12269 SYS      0g5b4q13v6gp4   enq: TX - row lock contention     1311          113                 1               17                       1                     17
也就是说使用online创建索引,存在历史记录的dml操作，将导致创建索引的session遭遇行锁等待！
SQL> select inst_id,sid,serial#,username,status from gv$session where inst_id=1 and sid=17;
INST_ID    SID SERIAL# USERNAME             STATUS
------- ------ ------- -------------------- ----------
      1     17   41003 SYS                  INACTIVE
      
SQL> alter system kill session '17,41003,@1' immediate;
System altered.

Session1  dml回滚！
SQL> r
  1* delete d1 where rownum=1
delete d1 where rownum=1
       *
ERROR at line 1:
ORA-03135: connection lost contact
Process ID: 37874
Session ID: 17 Serial number: 41003

SQL> select INST_ID,sid,serial#,USERNAME,SQL_ID,EVENT,(sysdate-LOGON_TIME)*86400 as "s",LAST_CALL_ET,BLOCKING_INSTANCE,BLOCKING_SESSION,FINAL_BLOCKING_INSTANCE,FINAL_BLOCKING_SESSION from gv$session where
   SID IN(19) and inst_id=2;
INST_ID    SID SERIAL# USERNAME             SQL_ID               EVENT                                s LAST_CALL_ET BLOCKING_INSTANCE BLOCKING_SESSION FINAL_BLOCKING_INSTANCE FINAL_BLOCKING_SESSION
------- ------ ------- -------------------- -------------------- ------------------------------ ------- ------------ ----------------- ---------------- ----------------------- ----------------------
      2     19   12269 SYS                  0g5b4q13v6gp4        enq: TX - row lock contention     1637          439                 2               30                       2                     33
Session3 
SQL> roll;
Rollback complete.

Session2
SQL> create index c_id on d1(object_id) online;
Index created.
      
SQL> set autotrace on
SQL> select count(*) from d1 where object_id=2;
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |    13 |     1   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE   |      |     1 |    13 |            |          |
|*  2 |   INDEX RANGE SCAN| C_ID |     1 |    13 |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------



问题 创建索引的过程中,是否会阻塞后续的dml操作
Session1
SQL> set timing on
SQL> create index c_id on d1(object_id);
Index created.
Elapsed: 00:00:03.12
Session2
SQL>  delete d1 where object_id>9000;
Session3
SQL> select INST_ID,sid,serial#,USERNAME,STATUS,MACHINE,SQL_ID,EVENT,(sysdate-LOGON_TIME)*86400 as "s",LAST_CALL_ET from gv$session where status='ACTIVE' and username is not null
INST_ID    SID SERIAL# USERNAME             STATUS     MACHINE              SQL_ID               EVENT                                s LAST_CALL_ET
------- ------ ------- -------------------- ---------- -------------------- -------------------- ------------------------------ ------- ------------
      2     19   12269 SYS                  ACTIVE     d2                   7ddkb1x0zhpp5        enq: TM - contention              2316            2  --存在阻塞
      2     37    3783 SYS                  ACTIVE     d2                   2ymxxw3mapxd9        PX Deq: Execution Msg                0            0
      2     43   42485 SYS                  ACTIVE     d2                   2ymxxw3mapxd9        PX Deq: Execute Reply             2042            0
      2    402    5717 SYS                  ACTIVE     d2                                        OFS idle                          3156         3156
      1     23   53420 SYS                  ACTIVE     d2                   2ymxxw3mapxd9        PX Deq: Execution Msg                0            0
      1    403   22969 SYS                  ACTIVE     d1                                        OFS idle                          3520         3520
      1    468   60995 SYS                  ACTIVE     d1                   2k397xzajspfu        db file single write               322            3

7 rows selected.
SQL>SELECT SQL_TEXT FROM V$SQL WHERE SQL_ID='7ddkb1x0zhpp5';
SQL_TEXT
--------------------------------
delete d1 where object_id>9000