触发器报错ORA-04091、ORA-06512、ORA-04088解决方案

create or replace trigger ia_sl  before update on po_invoice  referencing old as old_value new as new_value  for each row    declarecursor test_cursor is select ntaxmny, ia_bill_b.cbill_bid               from ia_bill_b,                    po_settlebill_b,                    po_invoice_b,                    ia_bill,                    po_invoice              where po_invoice.cinvoiceid = po_invoice_b.cinvoiceid                and ia_bill_b.csourcebillitemid =                    po_settlebill_b.csettlebill_bid                and po_settlebill_b.cinvoice_bid = po_invoice_b.cinvoice_bid                and ia_bill_b.cbillid = ia_bill.cbillid                and ia_bill.bestimateflag = 'N'                and po_invoice.cinvoiceid = :new_value.cinvoiceid;begin  for nx in test_cursor loop    update ia_bill_b       set vdef1 = nx.ntaxmny     where ia_bill_b.cbill_bid = nx.cbill_bid;  end loop;END;