mysql的exists与inner join 和 not exists与 left join 性能差别惊人
insert into t1(a1) select b1 from t2 where not exists(select 1 from t1 where t1.id = t2.r_id);
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如果t1的数据量很大时，性能会非常慢。经过实践，用以下方法能提高很多。
insert into t1(a1)select b1 from t2left join (select distinct t1.id from t1 ) t1 on t1.id = t2.r_id where t1.id is null;
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select * from t1 where exists(select 1 from t2 where t1.id=t2.r_id);
?替换为：
select t1.* from t1 inner join (select distinct r_id from t2) t2 on t1.id= t2.r_id
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这是实践的得出的结果。不知否有其他更好的方法，或则这个只是特例而已。?
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