sql优化的一些经验

/* 优化前 2.078s*/
SELECT 	COUNT(t.x_id)
FROM
order_detail t
WHERE
t.user_sid = 'xiaoming'
AND t.order_type NOT IN (
'A',
'B',
'C',
'D'
);

/* 优化后 0.091s */

SELECT COUNT(a.x_id) FROM (
SELECT DISTINCT(t.x_id)
FROM order_detail t
WHERE t.user_sid = 'xiaoming'
AND t.order_type='N'
UNION ALL
(
SELECT DISTINCT(t.x_id)
FROM order_detail t
WHERE t.user_sid = 'xiaoming'
AND t.order_type IS NULL
)
UNION ALL
(
SELECT DISTINCT(t.x_id)
FROM order_detail t
WHERE t.user_sid = 'xiaoming'
AND t.order_type = ''
)
) a