Postgres 排序不一致

Question

我遇到了一个问题，即 postgres 排序结果的排序不一致。我按多个字段排序：ORDER BY categories.position ASC、photos.display_priority

我注意到这一点是因为当您浏览网站时，结果会分页。我发现了一个案例，我从第 1 页转到第 2 页，在第 2 页的顶部，我看到了靠近第 1 页底部的照片。

这是我的第 1 页查询：

SELECT "photos".*
FROM "photos"
    INNER JOIN "categories" ON "categories"."id" = "photos"."category_id"
WHERE "photos"."category_id" IN (221, 633, 377, 216, 634)
    AND (photos.caption IS NOT NULL
        AND photos.category_id IS NOT NULL
        AND photos.rights IS NOT NULL
        AND photos.deleted IS NULL)
ORDER BY categories.position ASC, photos.display_priority DESC
LIMIT 25 OFFSET 0;

还有我的第 2 页查询：

SELECT "photos".*
FROM "photos"
    INNER JOIN "categories" ON "categories"."id" = "photos"."category_id"
WHERE "photos"."category_id" IN (221, 633, 377, 216, 634)
    AND (photos.caption IS NOT NULL
        AND photos.category_id IS NOT NULL
        AND photos.rights IS NOT NULL
        AND photos.deleted IS NULL)
ORDER BY categories.position ASC, photos.display_priority DESC
LIMIT 25 OFFSET 25;

当我尝试同时获取两个页面（偏移量 0，限制 50）并检查两组之间的阈值时，没有重复，这不足为奇。

SELECT "photos".*
FROM "photos"
    INNER JOIN "categories" ON "categories"."id" = "photos"."category_id"
WHERE "photos"."category_id" IN (221, 633, 377, 216, 634)
    AND (photos.caption IS NOT NULL
        AND photos.category_id IS NOT NULL
        AND photos.rights IS NOT NULL
        AND photos.deleted IS NULL)
ORDER BY categories.position ASC, photos.display_priority DESC
LIMIT 50 OFFSET 0;

我的查询有问题吗？是否有我不理解的限制和顺序的操作顺序？

Answer 1

听起来categories.position 和photos.display_priority 对于所有结果行来说并不是唯一的。当用于排序的值都相等时，数据库服务器不会指定行的顺序；可以按任意顺序返回它们，即使查询之间的表数据没有改变。

要获得一致的排序，您必须添加第三个排序键，保证对所有行都是唯一的，例如该特定行的标识值。