ORA-04091和Compound Trigger（Oracle 11g）

Trigger常见有两种:行（Row Trigger）和语句（Statement Trigger）还有：Instead of Trigger和Event trigger。例子1-Row Trigger：CREATE OR REPLACE TRIGGER client AFTERINSERT ON tt1 FOR EACH rowBEGIN dbms_application_info.set_client_info（userenv（"client_info"）+1 ）;END;例子2-Statement TriggerCREATE OR REPLACE TRIGGER client_1 AFTERINSERT ON tt1BEGIN dbms_application_info.set_client_info（userenv（"client_info"）-1 ）;END;ORA-04091错误
Tom Kyte有一篇文章很好的解释了ORA-04091。http://www.oracle.com/technetwork/issue-archive/2008/08-sep/o58asktom-101055.html部分摘抄如下：Suppose wehave a table that includes country currency combinations with a primarycurrency. The following is sample data:
CountryCurrency Primary_CurrencyUS USD YUS USN NUS USS NWe need toenforce the rule that at most one currency can be primary for a given country.We have a BEFORE UPDATE trigger on the above table for each row （usingautonomous transaction to avoid the mutating error） to check whether thecountry has any primary currency.That was allI needed to read. I knew they had a serious bug on their hands when Iread—paraphrasing:At most one currency can be primary （we have a constraint that crosses rows in the table）.
We have a . . . trigger.
We are using an autonomous transaction to avoid the mutating table error.The trigger would have looked something like this: SQL< create or replace 2 trigger currencies_trigger 3 before update on currencies 4 for each row 5 declare 6 PRAGMA AUTONOMOUS_TRANSACTION; 7 l_cnt number; 8 begin 9 select count（*） 10 into l_cnt 11 from currencies 12 where primary_currency="Y" 13 and country = :new.country; 14 if （ l_cnt < 1 ） 15 then 16 raise_application_error 17 （-20000, "only one allowed"）; 18 end if; 19 end; 20 /

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