Select one single row from table b for each row from table a I have two tables:

A(name VARCHAR2(20), type VARCHAR2(20)) 

and 

B (type VARCHAR2(20), ... (20 fields more))

I need to select a single row from B for each distinct name,type from A taking into account that table b contains millions of records and my current select runs for 20min+

SELECT *
FROM   (SELECT a.*,
               Row_number()
                 over (
                   PARTITION BY a.TYPE
                   ORDER BY a.TYPE) rn
        FROM   tablea a,
               tableb b
        WHERE  a.name = 'SEARCHED_NAME'
               AND b.TYPE = a.TYPE)
WHERE  rn = 1
How about this:

Select * 
from 
  tableB b
 inner join
  (select a.*, row_number() over (partition by a.type order by a.type) rn from tableA a) a1
    on b.type=a1.type and a1.name='SEARCHED_NAME' and a1.rn=1


PS. Not tested, as you didn't provide a script to create and populate the tables.