我有一张表progresses（目前包含数十万条记录）：
Column | Type | Modifiers
---------------+-----------------------------+---------------------------------------------------------
id | integer | not null default nextval('progresses_id_seq'::regclass)
lesson_id | integer |
user_id | integer |
created_at | timestamp without time zone |
deleted_at | timestamp without time zone |
Indexes:
"progresses_pkey" PRIMARY KEY, btree (id)
"index_progresses_on_deleted_at" btree (deleted_at)
"index_progresses_on_lesson_id" btree (lesson_id)
"index_progresses_on_user_id" btree (user_id)
和视图v_latest_progresses，其将查询最近progress的user_id和lesson_id：
SELECT DISTINCT ON (progresses.user_id, progresses.lesson_id)
progresses.id AS progress_id,
progresses.lesson_id,
progresses.user_id,
progresses.created_at,
progresses.deleted_at
FROM progresses
WHERE progresses.deleted_at IS …