Find top salary per department - is there a more efficient query? - postgresql

Question

I have a query that works but I suspect I'm doing this inefficiently. Is there a more elegant approach to find the top salary in each department and the employee that earns it?
I'm doing a cte to find the max salary per dept id and then join that up with the employee data by matching salary and dept id. I have code below to build/populate the tables and the query at the end.
CREATE TABLE employee (
emplid SERIAL PRIMARY KEY,
name VARCHAR NOT NULL,
salary FLOAT NOT NULL,
depid INTEGER
);
INSERT INTO employee (name, salary, depid)
VALUES
('Chris',23456.99,1),
('Bob',98756.34,1),
('Malin',34567.22,2),
('Lisa',34967.73,2),
('Deepak',88582.22,3),
('Chester',99487.41,3);
CREATE TABLE department (
depid SERIAL PRIMARY KEY,
deptname VARCHAR NOT NULL
);
INSERT INTO department (deptname)
VALUES
('Engineering'),
('Sales'),
('Marketing');
--top salary by department
WITH cte AS (
SELECT d.depid, deptname, MAX(salary) AS maxsal
FROM employee e
JOIN department d ON d.depid = e.depid
GROUP BY d.depid, deptname
)
SELECT cte.deptname, e.name, cte.maxsal
FROM cte
JOIN employee e ON cte.depid = e.depid
AND e.salary = cte.maxsal
ORDER BY maxsal DESC;
Here is the target result:
"Marketing" "Chester" "99487.41"
"Engineering" "Bob" "98756.34"
"Sales" "Lisa" "34967.73"

Answer 1

In Postgres this can solved using the distinct on () operator:
SELECT distinct on (d.depid) d.depid, deptname, e.name, e.salary AS maxsal
FROM employee e
JOIN department d ON d.depid = e.depid
order by d.depid, e.salary desc;
Or you can use a window function:
select depid, deptname, emp_name, salary
from (
SELECT d.depid,
deptname,
e.name as emp_name,
e.salary,
max(e.salary) over (partition by d.depid) AS maxsal
FROM employee e
JOIN department d ON d.depid = e.depid
) t
where salary = maxsal;
Online example: https://rextester.com/MBAF73582

Answer 2

You should have an index:
create index employee_depid_salary_desc_idx on employee(depid, salary desc);
And then use the following query that can use the index:
select
depid,
deptname,
(
select
emplid
from employees
where depid=department.depid
order by salary desc
limit 1
) as max_salaried_emplid
from department;
(A join for retrieving data from the emplid left as an exercise for the reader).