Various PostgreSQL questions

Version: 7.1release-3.potato.1 (Debian package of 7.1release-3)

Question 1:
Apparently using NUMERIC(10, 2) in a type definition (DECLARE) of
a stored procedure in PL/pgSQL doesn't work; FLOAT works instead.
Is this normal ?

Question 2:
How can I implement a constraint which would always ensure the
SUM(money_amount) WHERE type = 1 in a specified table is always
zero ?

I tried that:

CREATE FUNCTION f_ecriture_balance_check ()
RETURNS opaque
AS 'DECLARE amount FLOAT;
BEGIN
amount := 0;
SELECT SUM(montant_signe)
FROM ecriture
WHERE (lot = NEW.lot)
INTO amount;

IF (CAST(amount AS NUMERIC(10,2)) != - NEW.montant_signe) THEN
RAISE EXCEPTION ''Sum of ecritures in lot is not zero'';
END IF;

RETURN new;
END;'
LANGUAGE 'plpgsql';

-- TODO
-- - STATEMENT instead of ROW when supported by PostgreSQL
CREATE TRIGGER t_ecriture_balance_insert
BEFORE INSERT
ON ecriture
FOR EACH ROW
EXECUTE PROCEDURE f_ecriture_balance_check ();

Of course this is only for INSERT (UPDATE and DELETE are modified).
Should I use SET CONSTRAINTS ALL DEFERRED within a transaction ?
This function always fails: I assume it gets called for each of the
row and not for the final state at the end of the transaction.

Question 3:
The following works, once. The second time it doesn't work (in the
same session/backend, see below for the error).

-- Only inherited.
CREATE TABLE insert_temporary_table_base
(id SERIAL NOT NULL,
numero_compte INT4 NOT NULL,
libelle TEXT NOT NULL,
date_valeur DATE NOT NULL,
montant_signe NUMERIC(10, 2) NOT NULL,
UNIQUE(id), PRIMARY KEY(id));

CREATE TABLE ecriture(date_valeur DATE NOT NULL,
montant_signe NUMERIC(10, 2) NOT NULL,
utilisateur TEXT NOT NULL,
exercice TEXT NOT NULL);

CREATE FUNCTION f_insertion_lot(TEXT, TEXT, TEXT)
RETURNS INT4
AS 'DECLARE amount FLOAT;
BEGIN
INSERT INTO ecriture(utilisateur,
exercice,
date_valeur,
montant_signe)
SELECT $1, $3,
date_valeur,
montant_signe
FROM insert_temp
ORDER BY insert_temp.id;

RETURN 0; -- faking
END;'
LANGUAGE 'plpgsql';

BEGIN WORK;
CREATE TEMPORARY TABLE insert_temp ()
INHERITS(insert_temporary_table_base);

INSERT INTO insert_temp (numero_compte,
libelle,
date_valeur,
montant_signe)
VALUES (1000,
'Paiement Marc pour cours SQL',
'2002-04-26',
-245);

INSERT INTO insert_temp (numero_compte,
libelle,
date_valeur,
montant_signe)
VALUES (3000,
'Marc, cours SQL',
'2002-04-26',
200);

INSERT INTO insert_temp (numero_compte,
libelle,
date_valeur,
montant_signe)
VALUES (3010,
'Marc, frais déplacement',
'2002-04-26',
50);

SELECT f_insertion_lot('schaefer',
'insert_temp',
'Exercice 2002');

DROP TABLE insert_temp;
COMMIT WORK;

BEGIN WORK;
CREATE TEMPORARY TABLE insert_temp ()
INHERITS(insert_temporary_table_base);

INSERT INTO insert_temp (numero_compte,
libelle,
date_valeur,
montant_signe)
VALUES (1000,
'Paiement Marc pour cours SQL',
'2002-04-26',
-245);

INSERT INTO insert_temp (numero_compte,
libelle,
date_valeur,
montant_signe)
VALUES (3000,
'Marc, cours SQL',
'2002-04-26',
200);

INSERT INTO insert_temp (numero_compte,
libelle,
date_valeur,
montant_signe)
VALUES (3010,
'Marc, frais déplacement',
'2002-04-26',
50);

SELECT f_insertion_lot('schaefer',
'insert_temp',
'Exercice 2002');

DROP TABLE insert_temp;
COMMIT WORK;

The error is (at the second COMMIT):
CREATE
INSERT 633792 1
INSERT 633793 1
INSERT 633794 1
psql:test.sql:104: ERROR: Relation 633722 does not exist
psql:test.sql:106: NOTICE: current transaction is aborted, queries ignored until end of transaction block
*ABORT STATE*

Question 4:
Is it possible to parametrize the table of a SELECT ?

SELECT * FROM $1 # for example.

Thank you for any hint or pointers!