Postgres C extended data type definition - c++

Question

When dealing with the following problems, Postgres is a bit tricky to deal with more complex structures. I want to set up a two-dimensional array of structure, but I don't know how to make Postgres C support me to do so? Do anyone have any ideas?
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}
Sort the rows of each position of the array to get the fo.lowing structure. The result of the first row below is the first position of the num field column array, and so on.the count 4 refers to returning the first n sorted.
select my_func(contents, num, 4) from table;
expected result:
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)}
......
......
Thanks in advance.

Answer 1

I'm not sure why you need C extended data type, but the following will give you what you want and can be implemented as plpgsql function.
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;
UPDATE: Problem of the above may be undefined order of 'unnest'.
The following gives consistent relation between index and num, but need to write the size of num array explicitly.
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;