ORA-04091 tableT is mutating, trigger/function may not see it - sql

Question

create or replace trigger discount
after insert or update
on product
for each row
declare
newcost number;
quant number;
id number;
BEGIN
id:= :NEW.pid;
quant:= (30/100)*:NEW.req_quant;
newcost:= :NEW.pcost - (10/100)*:NEW.pcost;
if :NEW.act_quant>quant then
update product set pcost=newcost where pid=id;
end if;
dbms_output.put_line('success');
end;
while writing query insert into product values(107,20,20,1000);
i get an this error ORA-04091 tableT is mutating, trigger/function may not see it
my table is
CREATE TABLE "PRODUCT"
( "PID" NUMBER(5,0),
"ACT_QUANT" NUMBER(5,0),
"REQ_QUANT" NUMBER(5,0),
"PCOST" NUMBER(10,0),
PRIMARY KEY ("PID") ENABLE
)
After inserting or updating trigger must check that whether the actualquantity of product is greater than 30% of requaired quantity if it is true we need to give discount of 10% on that product

Answer 1

Don't literally update table which is just being modified (which caused trigger to fire) because trigger can't see it; that's the good, old mutating table error.
if :NEW.act_quant > quant then
:new.pcost := newcost; --> use this
-- update product set pcost=newcost where pid=id; --> not this
end if;
Though, the whole trigger can be shortened to
create or replace trigger discount
after insert or update on product
for each row
begin
:new.pcost := case when :new.act_quant > (30/100) * :new.req_quant then 0.9 * :new.pcost
else :new.pcost
end;
end;