Get last row from group, limit number of results in PostgreSQL - postgresql

Question

I have a table with records representing a log, I omit rest of the columns in this example.
The id-column is autoincrement, item_id represents an item in app.
I need to get the latest item_id, for example two or three
CREATE TABLE "log" (
"id" INT,
"item_id" INT
);
-- TRUNCATE TABLE "log";
INSERT INTO "log" ("id", "item_id") VALUES
(1, 1),
(2, 2),
(3, 1),
(4, 1),
(5, 3),
(6, 3);
Basic query will list all results, latest at the top:
SELECT *
FROM "log"
ORDER BY "id" DESC
id item_id
6 3
5 3
4 1
3 1
2 2
1 1
I would like to have just two (LIMIT 2) last item_ids with their id. Last means - inserted last (ORDER BY id).
id item_id
6 3
4 1
Last three would be
id item_id
6 3
4 1
2 2
Once an item_id is returned, it is not returned again. So LIMIT 4 would return only three rows because there are only three unique item_id.
I am probably missing something. I already tried various combinations of DISTINCT OF, GROUP BY, LIMIT etc.
UPDATE #1:
After I tested query by S-man (below), I found out that it works for the data I provided howerer it does not work in general, for another set of data (sequence of item_id A, B and A again.). Here is another data set:
TRUNCATE TABLE "log";
INSERT INTO "log" ("id", "item_id") VALUES
(1, 1),
(2, 2),
(3, 3),
(4, 3),
(5, 1),
(6, 3);
Data in DB, ordered by id desc:
id item_id
6 3
5 1
4 3
3 3
2 2
1 1
Expected result for last three item_id
6 3
5 1
2 2

Answer 1

Well, after three changes, now we come back to the very first idea:
Just take DISTINCT ON:
demo:db<>fiddle
SELECT
*
FROM (
SELECT DISTINCT ON (item_id) -- 1
*
FROM log
ORDER BY item_id, id DESC
) s
ORDER BY id DESC -- 2
LIMIT 2
Returns exact one record of an ordered group. You group is the item_id, the order is id DESC, so you get the highest id for each item_id
Reorder by id DESC (instead of the previously ordered item_id) and limit your query output.