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)100explain extended select count(distinct n) from t1;id select_type table type possible_keys key key_len ref rows filtered Extra1 SIMPLE t1 range n n 4 NULL 10 100.00 Using index for group-byWarnings:Note 1003 /* select#1 */ select count(distinct `test`.`t1`.`n`) AS `count(distinct n)` from `test`.`t1`drop table t1;