Yeah -- that's the part I'm having trouble with (assuming SCAN TABLE in the explain output means it is not using the constraint). Here's the query (well, a simplified version that still reproduces):

SELECT
    processes.pid,
    process_open_sockets.local_address,
    process_open_sockets.local_port,
    process_open_sockets.remote_address,
    process_open_sockets.remote_port
FROM processes
JOIN process_open_sockets ON process_open_sockets.pid = processes.pid
WHERE (processes.pid % 2) == 0;