PostgresQL window 在连续 ID 块上运行

Question

我有一个 table 具有部分连续的整数 ID，即有 1,2,3, 6,7,8, 10, 23,24,25,26.

这样的块

间隙大小是动态的
块的长度是动态的

我对从 table 中选择的简单解决方案感到头疼并包含一列，其中的值对应于相应块的第一个 id。

即像这样

select id, first(id) over <what goes here?> first from table;

结果应该如下所示

| id | first |
|----|-------|
| 1  | 1     |
| 2  | 1     |
| 3  | 1     |
| 6  | 6     |
| 7  | 6     |
| 8  | 6     |
| 10 | 10    |
| 23 | 23    |
| 24 | 23    |
| 25 | 23    |
| 26 | 23    |

之后，我可以将此专栏与 partition by window 函数子句很好地结合使用。

到目前为止我想出的总是看起来与此相似但没有成功：

WITH foo AS (
    SELECT LAG(id) OVER (ORDER BY id)  AS previous_id,
           id                          AS id,
           id - LAG(id, 1, id) OVER (ORDER BY id) AS first_in_sequence
    FROM table)
SELECT *,
       FIRST_VALUE(id) OVER (ORDER BY id) AS first
FROM foo
ORDER BY id;

定义自定义 postgres 函数也是一个可接受的table解决方案。

感谢任何建议，

马蒂

Answer 1

这是一个如何做到这一点的想法。隐式游标虽然效率不高。

create or replace function ff()
returns table (r_id integer, r_first integer)
language plpgsql as
$$
declare
  running_previous integer;
  running_id integer;
  running_first integer := null;
begin 
    for running_id in select id from _table order by id loop
        if running_previous is distinct from running_id - 1 then
            running_first := running_id;
        end if;
        r_id := running_id;
        r_first := running_first;
        running_previous := running_id;
        return next;
    end loop;
end
$$;
-- test
select * from ff() as t(id, first);

Answer 2

在 Postgres 中，您可以 create a custom aggregate. 示例：

create or replace function first_in_series_func(int[], int)
returns int[] language sql immutable
as $$ 
    select case 
        when [2] is distinct from - 1 then array[, ]
        else array[[1], ] end; 
$$;

create or replace function first_in_series_final(int[])
returns int language sql immutable
as $$
    select [1]
$$;

create aggregate first_in_series(int) (
    sfunc = first_in_series_func,
    finalfunc = first_in_series_final,
    stype = int[]
);

Db<>fiddle.

阅读文档：User-Defined Aggregates

PostgresQL window 在连续 ID 块上运行

PostgresQL window function over blocks of continuous IDs

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