如何在 PostgreSQL 中获取分区（窗口）的大小？

Question

使用窗口时，如何获取当前分区的大小（行数）？

例如，假设我有一个表，其中包含博客中帖子的评论。我想知道每个帖子的第一条评论、第二条评论、最后一条评论和评论数是什么（没有另一个子查询，我按 post 和 do 分组COUNT(*)）。

查询应类似于：

SELECT DISTINCT
    post_id.
    first_value(comment_text) OVER wnd AS first_comment,
    nth_value(comment_text, 2) OVER wnd AS second_comment,
    last_value(comment_text) OVER wnd AS last_comment,
    SOME_FUNCTION(comment_text) OVER wnd AS number_of_comments
FROM comments
WINDOW wnd AS (
    PARTITION BY post_id
    ORDER BY comment_created_at ASC
    ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
);

应该SOME_FUNCTION是什么？

Answer 1

一旦您意识到可以将它与 Windows 一起使用，它就在您的问题中：COUNT(*) OVER wnd AS number_of_comments将完成这项工作。

SELECT DISTINCT
    post_id.
    first_value(comment_text) OVER wnd AS first_comment,
    nth_value(comment_text, 2) OVER wnd AS second_comment,
    last_value(comment_text) OVER wnd AS last_comment,
    COUNT(*) OVER wnd AS number_of_comments
FROM comments
WINDOW wnd AS (
    PARTITION BY post_id
    ORDER BY comment_created_at ASC
    ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
);