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explain SELECT distinct t3.a FROM t3,t2,t1 WHERE t3.a=t1.b AND t1.a=t2.a;
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id select_type table type possible_keys key key_len ref rows Extra
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1 SIMPLE t1 ALL PRIMARY NULL NULL NULL 4 Using temporary
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SELECT distinct t3.a FROM t3,t2,t1 WHERE t3.a=t1.b AND t1.a=t2.a;