postgresql:suddivisione del periodo di tempo all'evento

Puoi fare UNION invece. In questo modo non generi righe non necessarie

SELECT country, start_date, 
       CASE WHEN event_date BETWEEN start_date AND end_date 
            THEN event_date - 1 
            ELSE end_date
       END AS end_date, event_date
  FROM table1
 UNION ALL
SELECT country, event_date, end_date, event_date
  FROM table1
 WHERE event_date BETWEEN start_date AND end_date
 ORDER BY country, start_date, end_date, event_date

Ecco un SQLFiddle demo

Uscita:

| country | start_date |   end_date | event_date |
|---------|------------|------------|------------|
|       A | 1960-01-01 | 1994-07-19 | 1994-07-20 |
|       A | 1994-07-20 | 1999-12-31 | 1994-07-20 |
|       B | 1926-01-01 | 1995-12-31 |     (null) |