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我有一個運行緩慢的更新語句,我很好奇如果將where條件移動到join子句可以提高性能。這裏的查詢:將where子句移動到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';
現在...對於一個非常大的表(數百萬條記錄),這樣做會更快嗎?
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中有多少行數有TYPE ='something'? T2中相應行的分數是否相同? T1中每行有多少行通常在T2? – Bohemian