寻求加快查询速度的方法，下面两者都花了很长时间，超过1秒，请评论如何让它变得更好！
查询1（1.03秒）：
SELECT id, Quantity AS poqty, 0 AS outqty
FROM po AS a
WHERE active
AND id NOT
IN (
SELECT id
FROM po_closed
)
解释结果：
> id select_type table type possible_keys key key_len ref rows Extra
> 1 PRIMARY a ALL NULL NULL NULL NULL 497495 Using where
> 2 SUBQUERY po_closed index id id 4 NULL 484399 Using index
查询2（1.39秒）
SELECT a.id, Quantity AS poqty, 0 AS outqty
FROM po AS a
LEFT OUTER JOIN po_closed AS b ON a.id = b.id
WHERE a.active
AND b.id IS NULL
解释结果：
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE a ALL NULL NULL NULL NULL 497495 Using where
1 SIMPLE b ref id id 4 a.id 1 Using where; Not exists; Using index
我也试过EXISTS，需要1.8秒。
谢谢，