INSERT INTO persona_followers(service_id, follower_id, valid_at) SELECT service_id, follower_id, NOW() FROM (SELECT DISTINCT service_id, follower_id FROM import) AS import WHERE NOT EXISTS(SELECT * FROM persona_followers WHERE import.service_id = persona_followers.service_id AND import.follower_id = persona_followers.follower_id);
I'm wondering if you have an unneeded level of nesting. (I don't know if it would make any difference, but it might).
INSERT INTO persona_followers(service_id, follower_id, valid_at) SELECT DISTINCT service_id, follower_id, now()
FROM import WHERE NOT EXISTS(SELECT * FROM persona_followers WHERE import.service_id = persona_followers.service_id AND import.follower_id = persona_followers.follower_id);