CREATE VIEW v_invoice_total_price_0
AS
 -- calculates total price for invoices with items
 SELECT
  id_invoice,
  SUM(price) AS total_price,
  SUM(vat) AS total_vat
 FROM
  t_invoice_item
 GROUP BY
  t_invoice_item.id_invoice
UNION
 -- ensures all invoices are in the list
 SELECT
   t_invoice.id_invoice,
   0 AS total_price,
   0 AS total_vat
 FROM
   t_invoice;


-- makes sure there is one single row per invoice
CREATE VIEW v_invoice_total_price
AS
SELECT
  id_invoice,
  SUM(total_price) AS total_price,
  SUM(total_vat) AS total_vat
FROM
  v_invoice_total_price_0
GROUP BY
  id_invoice;

SELECT inv.id_invoice 
      ,SUM(nvl(itm.price,0)) AS total_price 
      ,SUM(nvl(itm.vat,0))   AS total_vat
FROM   t_invoice      inv
      ,t_invoice_item itm
WHERE  inv.id_invoice = itm.id_invoice
GROUP BY inv.id_invoice;

SELECT /*+ NO_MERGE(tii) USE_HASH(tii) */ inv.id_invoice, NVL(tii.total_price,0) total_price, 
  NVL(tii.total_vat,0) total_vat
FROM t_invoice      inv,
  ( SELECT id_invoice, SUM(price) total_price, SUM(vat) total_vat
    FROM t_invoice_item
    GROUP BY id_invoice ) tii
WHERE inv.id_invoice = tii.id_invoice(+)

create index t_invoice_item_fk1 on invoice_item(invoice_id,price,vat);

select a.invoice_id
      ,sum(b.price) sum_price
      ,sum(b.vat) sum_vat
from  t_invoice a
     ,t_invoice_item b
where a.invoice_id = b.invoice_id(+)
/