两个会话分别只执行一个SQL会不会形成死锁

网友投稿 323 2023-12-18

两个会话分别只执行一个SQL会不会形成死锁

这篇文章给大家介绍两个会话分别只执行一个SQL会不会形成死锁，内容非常详细，感兴趣的小伙伴们可以参考借鉴，希望对大家能有所帮助。

1，问题

两个会话分别只执行一个SQL，可能形成死锁吗？

2，测试设想

对于一个大表（比如100万条记录），两个会话分别从一个大表的两端（头、尾）更新，就可能形成互相等待对方已占有资源的情况，从而形成死锁。

3，测试

3.1测试版本：

Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production

PL/SQL Release 11.2.0.3.0 - Production

CORE 11.2.0.3.0 Production

TNS for IBM/AIX RISC System/6000: Version 11.2.0.3.0 - Production

NLSRTL Version 11.2.0.3.0 - Production

3.2 创建测试用表

create table tmp_x(x int,y int);

插入数据：

insert into tmp_x

select rownum,rownum

from dual

connect by level<1e6+1;

创建索引：

create index idx_tmp_x_x on tmp_x(x);

收集统计信息：

begin

dbms_stats.gather_table_stats(user,TMP_X);

end;

3.3测试

--会话1

查看执行计划：

explain plan for update /*+ index_asc(t idx_tmp_x_x) */ tmp_x t set y=1 where x>1;

select * from table(dbms_xplan.display(null,null,Advanced));

Plan hash value: 4167283686

---------------------------------------------------------------------------------

---------------------------------------------------------------------------------

| 0 | UPDATE STATEMENT | | 999K| 9765K| 4340 (2)| 00:00:01 |

|* 2 | INDEX RANGE SCAN| IDX_TMP_X_X | 999K| 9765K| 2254 (2)| 00:00:01 |

---------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

2 - access("X">1)

Column Projection Information (identified by operation id):

-----------------------------------------------------------

2 - (upd=3; cmp=2) "T".ROWID[ROWID,10], "X"[NUMBER,22], "Y"[NUMBER,22]

执行SQL：

update /*+ index_asc(t idx_tmp_x_x) */ tmp_x t set y=1 where x>1;

--会话2

查看执行计划（使用提示index_desc，CARDINALITY）：

explain plan for update /*+ index_desc(t idx_tmp_x_x) CARDINALITY(t 1000) */ tmp_x t set y=2 where x<1e6;

select * from table(dbms_xplan.display(null,null,Advanced));

Plan hash value: 2352573976

--------------------------------------------------------------------------------------------

--------------------------------------------------------------------------------------------

| 0 | UPDATE STATEMENT | | 1000 | 10000 | 4340 (2)| 00:00:01 |

|* 2 | INDEX RANGE SCAN DESCENDING| IDX_TMP_X_X | 1000 | 10000 | 2254 (2)| 00:00:01 |

-------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

2 - access("X"<1e6)

filter("X"<1e6)

执行SQL：

update /*+ index_desc(t idx_tmp_x_x) CARDINALITY(t 1000) */ tmp_x t set y=2 where x<1e6;

---会话3

查看会话1,2的等待事件：

select sid,event,blocking_session from v$session where sid in (1894,2324);

SID EVENT BLOCKING_SESSION

---------- ----------------------------- ----------------

1894 SQL*Net message from client

2324 SQL*Net message from client

SQL> /

SID EVENT BLOCKING_SESSION

---------- ----------------------------- ----------------

1894 gc current request

2324 db file sequential read

SQL> /

SID EVENT BLOCKING_SESSION

---------- ----------------------------- ----------------

1894 db file scattered read

2324 gc current request

SQL> /

SID EVENT BLOCKING_SESSION

---------- ----------------------------- ----------------

1894 gc current multi block reques

2324 db file scattered read

SQL> /

SID EVENT BLOCKING_SESSION

---------- ----------------------------- ----------------

1894 gc current request

2324 gc current request

SQL> /

SID EVENT BLOCKING_SESSION

---------- ----------------------------- ----------------

1894 gc current request

2324 db file sequential read

SQL> /

SID EVENT BLOCKING_SESSION

---------- ----------------------------- ----------------

1894 db file sequential read

2324 db file scattered read

SQL> /

SID EVENT BLOCKING_SESSION

---------- ----------------------------- ----------------

1894 log buffer space

2324 log buffer space

SQL> /

SID EVENT BLOCKING_SESSION

---------- ----------------------------- ----------------

1894 gc current request

2324 gc current request

SQL> /

SID EVENT BLOCKING_SESSION

---------- ----------------------------- ----------------

1894 enq: TX - row lock contention

2324 enq: TX - row lock contention

SQL> /

SID EVENT BLOCKING_SESSION

---------- ----------------------------- ----------------