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drop table if exists t1; create table t1(n int not null, key(n)) delay_key_write = 1; select count(distinct n) from t1; count(distinct n) 100 explain extended select count(distinct n) from t1; id select_type table type possible_keys key key_len ref rows filtered Extra 1 SIMPLE t1 index NULL n 4 NULL 200 100.00 Using index Warnings: Note 1003 select count(distinct `test`.`t1`.`n`) AS `count(distinct n)` from `test`.`t1` drop table t1;