Slow SQL update when delayed_job table gets large

[brchristian]

I've been noticing my delayed_job workers going incredibly slowly (1 job per second), and looked at the logs to see what might be up. It seems like about 90%+ of the time per job is spent doing delayed_job bookkeeping, which seems like something must be amiss.

Here's the line in question:

SQL (811.3ms) UPDATE `delayed_jobs` SET `locked_at` = '2014-04-17 22:32:20', `locked_by` = 'host:b38f770a-f3f3-4b2a-8c66-7c8eebdb7fea pid:2' WHERE ((run_at <= '2014-04-17 22:32:20' AND (locked_at IS NULL OR locked_at < '2014-04-17 18:32:20') OR locked_by = 'host:b38f770a-f3f3-4b2a-8c66-7c8eebdb7fea pid:2') AND failed_at IS NULL) ORDER BY priority ASC, run_at ASC LIMIT 1

Am I somehow missing a db index or something? I'm guessing that this command isn't supposed to take 800ms!