1 drop table if exists t1;
2 create table t1(n int not null, key(n)) delay_key_write = 1;
3 select count(distinct n) from t1;
6 explain extended select count(distinct n) from t1;
7 id select_type table type possible_keys key key_len ref rows filtered Extra
8 1 SIMPLE t1 index NULL n 4 NULL 200 100.00 Using index
10 Note 1003 select count(distinct `test`.`t1`.`n`) AS `count(distinct n)` from `test`.`t1`