按机场（内部联接）过滤，并在UNION ALL之前进行聚合，以减少传递到最终聚合简化程序的数据集。具有UNION ALL的UNION ALL子查询应该比UNION ALL之后具有更大数据集的Join并行且运行速度更快。
SELECT f.airport, SUM(cnt) AS Total_Flights
FROM (
SELECT a.airport, COUNT(*) as cnt
FROM flights_stats f
INNER JOIN airports a ON f.Origin=a.iata AND a.country='USA'
WHERE Cancelled = 0 AND Month IN (3,4)
GROUP BY a.airport
UNION ALL
SELECT a.airport, COUNT(*) as cnt
FROM flights_stats f
INNER JOIN airports a ON f.Dest=a.iata AND a.country='USA'
WHERE Cancelled = 0 AND Month IN (3,4)
GROUP BY a.airport
) f
GROUP BY f.airport
ORDER BY Total_Flights DESC
LIMIT 10
;
调整mapjoin并启用并行执行：
set hive.exec.parallel=true;
set hive.auto.convert.join=true; --this enables map-join
set hive.mapjoin.smalltable.filesize=25000000; --size of table to fit in memory
602392714
清零编程群