为何在查看中使用此查询时速度较慢？

Question

我有这个疑问：

SELECT * FROM 
(
SELECT LASTNAME,NAME,STREET,PHONE,ID_CARD 
FROM PHONES1..TVPHONES 
UNION ALL 

SELECT LASTNAME,NAME,STREET,PHONE,ID_CARD 
FROM PHONES2..TVPHONES 
UNION ALL 

SELECT LASTNAME,NAME,STREET,PHONE,ID_CARD 
FROM PHONES3..TVPHONES 
) q 
WHERE ID_CARD = 'B5XXXXXXX' 

此查询需要0.03秒完成，但如果我转换成一个视图这一点，并执行相同的搜索：

ALTER VIEW [dbo].[BTPHONES] 
AS 

SELECT * FROM 
    (
    SELECT LASTNAME,NAME,STREET,PHONE,ID_CARD 
    FROM PHONES1..TVPHONES 
    UNION ALL 

    SELECT LASTNAME,NAME,STREET,PHONE,ID_CARD 
    FROM PHONES2..TVPHONES 
    UNION ALL 

    SELECT LASTNAME,NAME,STREET,PHONE,ID_CARD 
    FROM PHONES3..TVPHONES 
    ) q 

GO 

-- ------------------------ 

SELECT TOP 1 * FROM dbo.BTPHONES 
WHERE ID_CARD = 'B5XXXXXXX' 

需要大约3分钟！为什么需要这么长时间？

Plan01：

Plan02：

Answer 1

第二种情况是等价于：

SELECT TOP 1 * FROM (
    SELECT * FROM 
    (
    SELECT LASTNAME,NAME,STREET,PHONE,ID_CARD 
    FROM PHONES1..TVPHONES 
    UNION ALL 

    SELECT LASTNAME,NAME,STREET,PHONE,ID_CARD 
    FROM PHONES2..TVPHONES 
    UNION ALL 

    SELECT LASTNAME,NAME,STREET,PHONE,ID_CARD 
    FROM PHONES3..TVPHONES 
) q 
) 
WHERE ID_CARD = 'B5XXXXXXX' 

优化器可能没有意识到，它可能应用where子句t o每个内部选择。相反，它会创建整个表的联合，然后对结果执行全面扫描以应用where子句。