Postgres LIMIT/OFFSET 奇怪的行为

Question

我使用的是 PostgreSQL 9.6。我有一个这样的查询：

SELECT anon_1.id AS anon_1_id, anon_1.is_valid AS anon_1_is_valid, anon_1.first_name AS anon_1_first_name, anon_1.last_name AS anon_1_last_name,
anon_1.patronymic_name AS anon_1_patronymic_name,
anon_1.experience AS anon_1_experience, anon_1.user_id AS anon_1_user_id, anon_1.rating_points as rating_points

FROM (SELECT DISTINCT ON (doctors.id) doctors.id AS id, doctors.created_at AS created_at, doctors.updated_at AS updated_at, doctors.is_valid AS is_valid, doctors.pretty_url AS pretty_url, doctors.first_name AS first_name, doctors.last_name AS last_name, doctors.patronymic_name AS patronymic_name, doctors.phone AS phone, doctors.birthday AS birthday, doctors.avatar AS avatar, doctors.experience AS experience, doctors.science_degree AS science_degree, doctors.sex_id AS sex_id, doctors.yclients_staff_id AS yclients_staff_id, doctors.user_id AS user_id, doctor_has_specialties.rating_points AS rating_points, clinic_branch_has_doctors.price AS price, clinic_branch_has_doctors.doctor_type AS doctor_type, clinic_branch_has_doctors.is_house_call AS is_house_call, clinic_branch_has_doctors.house_call_price AS house_call_price 
FROM doctors
      JOIN doctor_has_specialties ON doctors.id = doctor_has_specialties.doctor_id 
      JOIN clinic_branch_has_doctors ON doctor_has_specialties.id = clinic_branch_has_doctors.doctor_has_specialty_id 
      JOIN clinic_branches ON clinic_branches.id = clinic_branch_has_doctors.clinic_branch_id 
      JOIN city_areas ON city_areas.id = clinic_branches.city_area_id 
      JOIN cities ON cities.id = city_areas.city_id 
WHERE doctors.is_valid = true 
      AND clinic_branch_has_doctors.is_valid = true 
      AND clinic_branches.is_valid = true 
      AND doctor_has_specialties.specialty_id = 1
      AND cities.id = 1) AS anon_1 ORDER BY anon_1.rating_points DESC 
 LIMIT 100 OFFSET 0

这里查询最重要的部分是最后一个，LIMIT 和OFFSET。 当我运行这个查询时，我的所有数据最多 100 行。一切都很好不是。这是pgAdmin的截图：

此处通知 id 为 20 的行。 现在，如果我尝试偏移 10，我会按预期从第 11 行获取我的数据，这是 id 为 22 的对象。一切都很好。 但是如果我尝试 OFFSET 10 LIMIT 10，我会得到奇怪的结果。主要是 id 20 的行也出现了，但它不应该出现。这是屏幕截图：

不知道这有什么问题。是 Postgres 错误吗？或者我做错了什么。

Answer 1

这里没有“错误”在起作用。您指定了以下顺序，在应用 LIMIT 和 OFFSET 时使用：

ORDER BY anon_1.rating_points DESC

但是，如果两个或多个记录与相同的rating_points 值相关联，Postgres 不保证顺序是什么。这就是为什么您会看到anon_id_1 或20 的用户显然在四处走动。 Postgres 没有做错任何事；它尊重您通过rating_points 订购的请求，但您从未告诉它遇到领带时该怎么做。

要解决此问题，您可以向ORDER BY 添加第二个条件：

ORDER BY
    anon_1.rating_points DESC,
    anon_id_1

这将打破关于排序的关系，并且假设anon_id_1 是主键，在进行此更改后结果看起来很稳定。