Aggregate function for corresponding row - postgresql

Question

I have the following table with a combined primary key of id and ts to implement historization:
create table "author" (
"id" bigint not null,
"ts" timestamp not null default now(),
"login" text unique not null,
primary key ("id", "ts")
);
Now I am interested only in the latest login value. Therefor I group by id:
select "id", max("ts"), "login" from "author" group by "id";
But this throws an error: login should be used in an aggregate function.
id and max("ts") uniquely identify a row because the tupple (id, ts) is the primary key. I need the login which matches the row identified by id and max("ts").
I can write a sub-select to find the login:
select ao."id", max(ao."ts"),
(select ai.login from "author" ai
where ao."id" = ai."id" and max(ao."ts") = ai.ts)
from "author" ao
group by "id";
This works but it is quite noisy and not very clever, because it searches the whole table although searching the group would be sufficient.
Does an aggregate function exist, which avoids the sub-select and gives me the remaining login, which belongs to id and max("ts")?

Answer 1

You have to identify the correct key to get the value you like from the table.
The correct key is:
select "id", max("ts") from "author" group by "id";
And using this to get the login you want:
select a1."id", a1.ts, a1.login
from "author" a1
inner join (select "id", max("ts") maxts, "login" from "author" group by "id") a2
ON a1.id = a2.id AND a1.ts = a2.maxts;
Alternatively using window functions:
SELECT "id", "ts", login
FROM (
select "id", "ts", CASE WHEN "ts" = max("ts") OVER (PARTITION BY "id") THEN 1 ELSE 0 END as isMax, "login" from "author" group by "id"
) dt
WHERE isMax = 1
There's a few other ways to skin this cat, but that's basically the gist.