SQL Server中的查询优化

Question

SELECT 
    T2.Entity1Id, T1.Entity1Id  
FROM 
    T1  
FULL OUTER JOIN 
    T2 ON T1.c2 = T2.c2 AND T1.c1 = T2.c1 AND T1.c3 = 1
WHERE 
    ((T1.c1 = 123 ) OR (T2.c1 = 123))  
    AND (T1.c3 = 1 OR T1.c3 IS NULL)

上面的查询在SQL Server 2014中耗时12秒，您有什么想法可以调整查询？ 在C1，C2，C3列上有索引。

观察：在上面的查询中，当我从OR中删除条件时（即

SELECT  
    T2.Entity1Id, T1.Entity1Id  
FROM 
    T1  
FULL OUTER JOIN 
    T2 ON T1.c2 = T2.c2 AND T1.c1 = T2.c1  AND T1.c3 = 1
WHERE 
    (T1.c1 = 123) AND (T1.c3 = 1 OR T1.c3 IS NULL)

然后在0秒内返回结果。

每个表有大约500'000条记录。

Answer 1

首先，最终条件(T1.c3 = 1 OR T1.c3 IS NULL)是多余的。 给定join条件，这些是唯一可能的值。 因此，查询为：

SELECT T2.Entity1Id, T1.Entity1Id  
FROM T1 FULL OUTER JOIN
     T2
     ON T1.c2 = T2.c2 AND T1.c1 = T2.c1  AND T1.c3 = 1
WHERE (T1.c1 = 123 ) OR (T2.c1 = 123)

如果这样做的性能不好，请考虑将其分为两个查询：

SELECT T2.Entity1Id, T1.Entity1Id  
FROM T1 LEFT JOIN
     T2
     ON T1.c2 = T2.c2 AND T1.c1 = T2.c1  AND T1.c3 = 1
WHERE T1.c1 = 123
UNION 
SELECT T2.Entity1Id, T1.Entity1Id  
FROM T2 LEFT JOIN
     T1
     ON T1.c2 = T2.c2 AND T1.c1 = T2.c1  AND T1.c3 = 1
WHERE T2.c1 = 123

有时，单独子查询的优化要比full outer join的优化好得多。