mysql> SELECT COUNT(DISTINCT uid_entered) FROM 2_l10n_community_translation GROUP BY uid_entered;
+-----------------------------+
| COUNT(DISTINCT uid_entered) |
+-----------------------------+
|                           1 | 
|                           1 | 
+-----------------------------+
2 rows in set (7.52 sec)


mysql> explain SELECT COUNT(DISTINCT uid_entered) FROM 2_l10n_community_translation GROUP BY uid_entered;
+----+-------------+------------------------------+------+---------------+------+---------+------+---------+----------------+
| id | select_type | table                        | type | possible_keys | key  | key_len | ref  | rows    | Extra          |
+----+-------------+------------------------------+------+---------------+------+---------+------+---------+----------------+
|  1 | SIMPLE      | 2_l10n_community_translation | ALL  | NULL          | NULL | NULL    | NULL | 5369560 | Using filesort | 
+----+-------------+------------------------------+------+---------------+------+---------+------+---------+----------------+
1 row in set (0.00 sec)

mysql> create index l10nuid_entered on 2_l10n_community_translation(uid_entered);
Query OK, 5369560 rows affected (2 min 30.20 sec)
Records: 5369560  Duplicates: 0  Warnings: 0

mysql> explain SELECT COUNT(DISTINCT uid_entered) FROM 2_l10n_community_translation GROUP BY uid_entered;
+----+-------------+------------------------------+-------+---------------+-----------------+---------+------+---------+-------------+
| id | select_type | table                        | type  | possible_keys | key             | key_len | ref  | rows    | Extra       |
+----+-------------+------------------------------+-------+---------------+-----------------+---------+------+---------+-------------+
|  1 | SIMPLE      | 2_l10n_community_translation | index | NULL          | l10nuid_entered | 4       | NULL | 5369560 | Using index | 
+----+-------------+------------------------------+-------+---------------+-----------------+---------+------+---------+-------------+
1 row in set (0.00 sec)

mysql> SELECT COUNT(DISTINCT uid_entered) FROM 2_l10n_community_translation GROUP BY uid_entered;
+-----------------------------+
| COUNT(DISTINCT uid_entered) |
+-----------------------------+
|                           1 | 
|                           1 | 
+-----------------------------+
2 rows in set (2.96 sec)