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	Extra
1	SIMPLE	t1	index	NULL	n	4	NULL	200	Using index
Warnings:
Note	1003	select high_priority count(distinct test.t1.n) AS `count(distinct n)` from test.t1
drop table t1;