PostgresQL window function over blocks of continuous IDs - postgresql

Question

I have a table with partially consecutive integer ids, i.e. there are blocks such as 1,2,3, 6,7,8, 10, 23,24,25,26.
the gap size is dynamic
the length of the blocks is dynamic
I am breaking my head about a simple solution that selects from the table
and includes a column where the value corresponds to the first id of the respective block.
I.e. something like this
select id, first(id) over <what goes here?> first from table;
The result should look as following
| id | first |
|----|-------|
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
| 6 | 6 |
| 7 | 6 |
| 8 | 6 |
| 10 | 10 |
| 23 | 23 |
| 24 | 23 |
| 25 | 23 |
| 26 | 23 |
Afterwards i could use this column nicely with the partition by window function clause.
What I came up with so far always looked similar to this and didn't succeed:
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;
Defining a custom postgres function would also be an acceptable solution.
Thanks for any advice,
Marti

Answer 1

In Postgres you can create a custom aggregate. Example:
create or replace function first_in_series_func(int[], int)
returns int[] language sql immutable
as $$
select case
when $1[2] is distinct from $2- 1 then array[$2, $2]
else array[$1[1], $2] end;
$$;
create or replace function first_in_series_final(int[])
returns int language sql immutable
as $$
select $1[1]
$$;
create aggregate first_in_series(int) (
sfunc = first_in_series_func,
finalfunc = first_in_series_final,
stype = int[]
);
Db<>fiddle.
Read in the docs: User-Defined Aggregates

Answer 2

Here is an idea how this could be done. An implicit cursor is not horribly efficient though.
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);