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我有一个运行缓慢的更新语句,我很好奇如果将where条件移动到join子句可以提高性能。这里的查询:将where子句移动到join子句是否可以提高性能?
update T1 inner join (select ID, GROUP_CONCAT(x) as X from T3 group by ID) as T2
on T1.ID=T2.ID set T1.X=T2.X where T1.TYPE='something';
现在...对于一个非常大的表(数百万条记录),这样做会更快吗?
update T1 inner join (select ID, GROUP_CONCAT(x) as X from T3 group by ID) as T2
on T1.ID=T2.ID and T1.TYPE='something' set T1.X=T2.X;
T1中有多少行数有TYPE ='something'? T2中相应行的分数是否相同? T1中每行有多少行通常在T2? – Bohemian