初版

build/lib/mysql/mysql-test/r/bench_count_distinct.result

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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;