Explain Query (mysql)

EXPLAIN SELECT `subscription`.`subscription_id` AS `t1_c0`, `subscription`.`user_id` AS `t1_c1`, `subscription`.`drop_instance_id` AS `t1_c2`, `subscription`.`basket_id` AS `t1_c3`, `subscription`.`created` AS `t1_c4`, `subscription`.`active` AS `t1_c5`, `subscription`.`start_date` AS `t1_c6`, `subscription`.`type` AS `t1_c7`, `subscription`.`state` AS `t1_c8`, `subscription`.`became_superlufavore_on` AS `t1_c9`, `subscription`.`signup_variation` AS `t1_c10`, `subscription`.`subscriptions_order_prepopulation_method` AS `t1_c11` FROM `subscriptions` `subscription` WHERE (`subscription`.`user_id`='308842')
id select_type table partitions type possible_keys key key_len ref rows filtered Extra
1 SIMPLE subscription const uniq_sub_per_user, idx_user_id uniq_sub_per_user 4 const 1 100.00