Mutating table (urgent)

Hi,

I'm having a big throuble with postgreSQL, called "mutating table"

This problem also happen with Oracle,
but we found a soluction creating packagers.

In first.
I would like to explain that this exemple is just a exemple.
Don't try to understand its utility.

Step to emulation the error:

1) Creating a principal table and a child table
CREATE TABLE PAI(CODPAI INTEGER PRIMARY KEY, NOMPAI VARCHAR(30));
CREATE TABLE FILHO(CODPAI INTEGER, CODFIL SERIAL, NOMFIL VARCHAR(30), PRIMARY KEY (CODPAI, CODFIL));

2) Creating constraint between principal and child table
ALTER TABLE FILHO ADD CONSTRAINT FK_PAI_FILHO FOREIGN KEY (CODPAI) REFERENCES PAI (CODPAI);

3) Creating trigger on principal table
CREATE OR REPLACE FUNCTION TR_AIPAI() RETURNS OPAQUE AS '
BEGIN
INSERT INTO FILHO (CODPAI, NOMFIL) VALUES (NEW.CODPAI, ''FILHO DO PAI''||NEW.CODPAI);
RETURN NULL;
END;
' language 'plpgsql';

CREATE TRIGGER AIPAI AFTER INSERT ON PAI FOR EACH ROW EXECUTE PROCEDURE TR_AIPAI();

If we taking a insert on principal table it will insert on child table,
until here, thats all right...

4) Create a trigger on child table that make a select on principal table
CREATE OR REPLACE FUNCTION TR_SELECT_PAI() RETURNS OPAQUE AS'
DECLARE I INTEGER;
BEGIN
SELECT COUNT(*) INTO I FROM PAI;
RETURN NULL;
END;
'language 'plpgsql';

CREATE TRIGGER BIFILHO BEFORE INSERT ON FILHO FOR EACH ROW EXECUTE PROCEDURE TR_SELECT_PAI();

In this moment when we insert a register on principal table
it don't insert the register on child table, neither send any error message.

If anybody can help me, I'll be thanks for ever.

Cristian Custodio