這篇文章給大家介紹兩個會話分別只執(zhí)行一個SQL會不會形成死鎖，內(nèi)容非常詳細，感興趣的小伙伴們可以參考借鑒，希望對大家能有所幫助。
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1，問題
兩個會話分別只執(zhí)行一個SQL，可能形成死鎖嗎？
2，測試設(shè)想
對于一個大表（比如100萬條記錄），兩個會話分別從一個大表的兩端（頭、尾）更新，就可能形成互相等待對方已占有資源的情況，從而形成死鎖。
3，測試
3.1測試版本：
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 創(chuàng)建測試用表
create table tmp_x(x int,y int);
插入數(shù)據(jù)：
insert into tmp_x
select rownum,rownum
from dual
connect by level<1e6+1;
創(chuàng)建索引：
create index idx_tmp_x_x on tmp_x(x);
收集統(tǒng)計信息：
begin
dbms_stats.gather_table_stats(user,'TMP_X');
end;
/
3.3測試
--會話1
查看執(zhí)行計劃：
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
---------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------
| 0 | UPDATE STATEMENT | | 999K| 9765K| 4340 (2)| 00:00:01 |
| 1 | UPDATE | TMP_X | | | | |
|* 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]
執(zhí)行SQL：
update /*+ index_asc(t idx_tmp_x_x) */ tmp_x t set y=1 where x>1;
--會話2
查看執(zhí)行計劃（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
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | UPDATE STATEMENT | | 1000 | 10000 | 4340 (2)| 00:00:01 |
| 1 | UPDATE | TMP_X | | | | |
|* 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)
執(zhí)行SQL：
update /*+ index_desc(t idx_tmp_x_x) CARDINALITY(t 1000) */ tmp_x t set y=2 where x<1e6;
---會話3
查看會話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
---------- ----------------------------- ----------------
1894 enq: TX - row lock contention 2324
2324 enq: TX - row lock contention 1894
SQL> /
SID EVENT BLOCKING_SESSION
---------- ----------------------------- ----------------
1894 enq: TX - row lock contention 2324
2324 enq: TX - row lock contention 1894
SQL> /
SID EVENT BLOCKING_SESSION
---------- ----------------------------- ----------------
1894 enq: TX - row lock contention 2324
2324 enq: TX - row lock contention 1894
SQL> /
SID EVENT BLOCKING_SESSION
---------- ----------------------------- ----------------
1894 enq: TX - row lock contention 2324
2324 enq: TX - row lock contention
SQL> /
SID EVENT BLOCKING_SESSION
---------- ----------------------------- ----------------
1894 enq: TX - row lock contention 2324
2324 enq: TX - row lock contention
SQL> /
SID EVENT BLOCKING_SESSION
---------- ----------------------------- ----------------
1894 enq: TX - row lock contention
2324 SQL*Net message from client
--會話1
update /*+ index_asc(t idx_tmp_x_x) */ tmp_x t set y=1 where x>1
*
ERROR at line 1:
ORA-00060: deadlock detected while waiting for resource
3.4測試結(jié)論
兩個會話分別只執(zhí)行一個SQL，也可能形成死鎖。
關(guān)于兩個會話分別只執(zhí)行一個SQL會不會形成死鎖就分享到這里了，希望以上內(nèi)容可以對大家有一定的幫助，可以學(xué)到更多知識。如果覺得文章不錯，可以把它分享出去讓更多的人看到。
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