sql - 条件 IF 中的查询速度变慢

Question

我注意到以下命令在 SQL Server (2008 R2) 中运行需要 32 秒

IF  ((SELECT COUNT(*) FROM view1) != 0)
    OR ((SELECT COUNT(*) FROM view2) != 0)
    OR ((SELECT COUNT(*) FROM view3) != 0)
    OR ((SELECT COUNT(*) FROM view4) != 0)
    OR ((SELECT COUNT(*) FROM view5) != 0)
    OR ((SELECT COUNT(*) FROM view6) != 0)
        PRINT 'HI'

然而，下面的内容只花了4秒

SELECT 
    (SELECT COUNT(*) FROM view1)
    , (SELECT COUNT(*) FROM view2)
    , (SELECT COUNT(*) FROM view3)
    , (SELECT COUNT(*) FROM view4)
    , (SELECT COUNT(*) FROM view5)
    , (SELECT COUNT(*) FROM viwe6)

我已经通过执行 SELECT COUNT(1)... + SELECT... != 0 设法优化了条件，这样需要 4 秒，但查看执行计划并没有产生任何特别值得注意的结果。没有太多机会在​​谷歌上搜索“OR”这个词。

有人可以分享一下 SQL Server 优化器可能在背后做什么吗？

编辑:IF EXISTS 尝试花费了 38 秒。

IF EXISTS ((SELECT 1 FROM view1)
    UNION (SELECT 1 FROM view2)
    UNION (SELECT 1 FROM view3)
    UNION (SELECT 1 FROM view4)
    UNION (SELECT 1 FROM view5)
    UNION (SELECT 1 FROM view6))
        PRINT 'HI'

EDIT2:当前的 5 秒方法。

IF  (SELECT COUNT(1) FROM view1)
  + (SELECT COUNT(1) FROM view2)
  + (SELECT COUNT(1) FROM view3)
  + (SELECT COUNT(1) FROM view4)
  + (SELECT COUNT(1) FROM view5)
  + (SELECT COUNT(1) FROM viwe6) != 0
        PRINT 'HI'

EDIT3:比较 20 页大小的查询计划后 - 看起来加速主要是由于底层 View 在连接之前执行部分聚合，而不是先执行连接然后聚合。

Answer 1

使用 TOP 1 语句:

例如:

DECLARE @CONTROL INT;
WITH CTE AS (
SELECT TOP 1 col FROM view1
UNION ALL
SELECT TOP 1 col FROM view2
UNION ALL
SELECT TOP 1 col FROM view3
UNION ALL
SELECT TOP 1 col FROM view4
UNION ALL
SELECT TOP 1 col FROM view5
UNION ALL
SELECT TOP 1 col FROM view6)

SELECT @CONTROL = COUNT(col) FROM CTE

IF @CONTROL != 0
PRINT 'HI'