Postgres C 扩展数据类型定义

Question

在处理以下问题时，Postgres 处理更复杂的结构有点棘手。我想建立一个二维数组的结构，但不知道如何让Postgres C支持我这样做？有人有什么想法吗？

Table
    id     contents(text)              num(double)
    1       I love you.            {1,3,4,5,6,7,8,10}
    2       why do it?             {3,4,2,11,12,33,44,15}
    3       stopping.              {22,33,11,15,14,22,11,55}
    4       try it again.          {15,12,11,22,55,21,31,11}

对数组中每个位置的行进行排序，得到如下结构。下面第一行的结果是num字段列数组的第一个位置，以此类推。count 4是指返回排序后的前n个。

select my_func(contents, num, 4) from table;

预期结果：

                           result
{('stopping.', 22), ('try it again.', 15), ('why do it?', 3), ('I love you.', 1)}
{('stopping.', 33), ('try it again.', 12), ('why do it?', 4), ('I love you.', 3)}
{('stopping.', 11), ('try it again.', 11), ('I love you.', 4), ('why do it?', 2)}
......
......

提前致谢。

Answer 1

我不知道你为什么需要 C 扩展数据类型，但是下面会给你你想要的并且可以实现为 plpgsql 函数。

WITH t1 AS (
  SELECT id, contents, unnest (num) AS n FROM table
),
t2 AS (
  SELECT id, contents, n,
         row_number () OVER (PARTITION BY id ORDER BY id) AS o
         FROM t1 ORDER BY o ASC, n DESC, id ASC
),
t3 AS (
  SELECT array_agg (ROW (contents, n)) AS a, o
         FROM t2 GROUP BY o ORDER BY o
)
SELECT array_agg (a ORDER BY o) FROM t3;

更新：上述问题可能是“unnest”的未定义顺序。 下面给出 index 和 num 的一致关系，但需要明确写出 num 数组的大小。

WITH RECURSIVE t1 (i, id, contents, num) AS (
  SELECT 1, id, contents, num[1] FROM table
  UNION ALL
  SELECT t1.i + 1, table.id, table.contents, table.num[t1.i + 1]
         FROM t1, table
         WHERE t1.id = table.id AND t1.i < 8 -- put here size of array
),
t2 (i, d) AS (
  SELECT i, array_agg (ROW (contents, num) ORDER BY num DESC)
         FROM t1 GROUP BY i
)
SELECT array_agg (d ORDER BY i) FROM t2;