how to store an array of values into a variable?

Rule of thumb: if you’re looping in SQL there’s probably a better way.

You want to set state = 'down' until you have a certain number of idle servers.

We can do this in a single statement. Use a Common Table Expression to query your idle servers and feed that to an update from. If you do this a lot you can turn the CTE into a view.

But we need to limit how many we take down based on how many idle servers there are. We can do that with a limit. But update from doesn’t take a limit, so we need a second CTE to limit the results.

Here I’ve hard coded company_id 1 and p_num_min_idle_servers 2.

with idle_servers as (
  select id
  from server
  where server.company_id=1
    and connection_count=0
    and state = 'up'
),
idle_servers_to_take_down as (
  select id
  from idle_servers
  -- limit doesn't work in update from
  limit (select count(*) - 2 from idle_servers)
)
update server
set state = 'down'
from idle_servers_to_take_down
where server.id = idle_servers_to_take_down.id

This has the advantage of being done in one statement avoiding race conditions without having to lock the whole table.

Try it.