mutating trigger

hi everybody
can anyone explain me what is mutating trigger.

Hi, a mutating trigger occurs in oracle when the table that originally fired the triggering event is being accessed in the body of the trigger code, directly or implicitly in a procedure, or in a nested trigger called from the trigger body.

This action is forbidden because the table is in middle of a transaction, and referencing the same table again in the middle of the updating action causes the trigger to mutate. For example:

sql>create table t1 (x int);

sql>create table t2 (x int);

sql>insert into t1 values (1);

sql>select * from t1;

sql>select * from t2;

sql>create or replace trigger t_trigger

after insert on t1

for each row

declare

i pls_integer;

begin

select count(*) into i from t1;

insert into t2 values (i);

end; /

sql>insert into t1 values (1);

sql>select count(*) from t1;

sql>select count(*) from t2;

if u run this one u will get the mutating error, ora-04091.

Cheers, regards

rsy