sql-server - 为什么在视图中使用此查询会变慢？

Question

我有这个查询：

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 秒才能完成，但如果我将其转换为视图并执行相同的搜索：

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'

大约需要3分钟！！为什么需要这么长时间？

计划01：

计划02：

Answer 1

第二种情况等价于：

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以将 where 子句应用于每个内部选择。相反，它创建整个表的联合，然后对结果进行全面扫描以应用 where 子句。