ORA-04091和Compound Trigger(Oracle 11g)

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 row

BEGIN

 dbms_application_info.set_client_info(userenv('client_info')+1 );

END;

例子2-Statement Trigger

CREATE OR REPLACE TRIGGER client_1 AFTERINSERT ON tt1

BEGIN

 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_Currency

US      USD      Y

US      USN      N

US      USS      N

We 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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