我有两个表,每个表约有 200,000 行。我已经运行了下面的查询,但运行了一个多小时后仍然没有完成。对此有何解释?
SELECT
dbo.[new].[colom1],
dbo.[new].[colom2],
dbo.[new].[colom3],
dbo.[new].[colom4],
dbo.[new].[Value] as 'nieuwe Value',
dbo.[old].[Value] as 'oude Value'
FROM dbo.[new]
JOIN dbo.[old]
ON dbo.[new].[colom1] = dbo.[old].[colom1]
and dbo.[new].[colom2] = dbo.[old].[colom2]
and dbo.[new].[colom3] = dbo.[old].[colom3]
and dbo.[new].[colom4] = dbo.[old].[colom4]
where dbo.[new].[Value] <> dbo.[old].[Value]
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最佳答案
看起来,对于单列上的等式连接,连接键中具有 NULL 值的行将被过滤掉,但对于多列上的连接,情况并非如此。
结果,哈希连接复杂度从 O(N) 变为 O(N^2)。
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在这种情况下,我想推荐 Paul White 就类似问题撰写的一篇精彩文章 - Hash Joins on Nullable Columns
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我已经生成了这个用例的一个小型模拟,我鼓励您测试您的解决方案。
create table mytab1 (c1 int null,c2 int null)
create table mytab2 (c1 int null,c2 int null)
;with t(n) as (select 1 union all select n+1 from t where n < 10)
insert into mytab1 select null,null from t t0,t t1,t t2,t t3,t t4
insert into mytab2 select null,null from mytab1
insert into mytab1 values (111,222);
insert into mytab2 values (111,222);
<小时/>
select * from mytab1 t1 join mytab2 t2 on t1.c1 = t2.c1 and t1.c2 = t2.c2
<小时/>
对于 OP 查询,我们应该删除任何连接键列中具有 NULL 值的行。
SELECT
dbo.[new].[colom1],
dbo.[new].[colom2],
dbo.[new].[colom3],
dbo.[new].[colom4],
dbo.[new].[Value] as 'nieuwe Value',
dbo.[old].[Value] as 'oude Value'
FROM dbo.[new]
JOIN dbo.[old]
ON dbo.[new].[colom1] = dbo.[old].[colom1]
and dbo.[new].[colom2] = dbo.[old].[colom2]
and dbo.[new].[colom3] = dbo.[old].[colom3]
and dbo.[new].[colom4] = dbo.[old].[colom4]
where dbo.[new].[Value] <> dbo.[old].[Value]
and dbo.[new].[colom1] is not null
and dbo.[new].[colom2] is not null
and dbo.[new].[colom3] is not null
and dbo.[new].[colom4] is not null
and dbo.[old].[colom1] is not null
and dbo.[old].[colom2] is not null
and dbo.[old].[colom3] is not null
and dbo.[old].[colom4] is not null
关于执行 20 万行 SQL 查询需要一个多小时,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/41302138/