fix: Use a good index for dequeue (#61)

migrations/20240202003133_better_dequeue_index.sql

@@ -0,0 +1,10 @@
+-- Dequeue is not hitting this index, so dropping is safe this time.
+DROP INDEX idx_queue_scheduled_at;
+
+/*
+Partial index used for dequeuing from job_queue.
+
+Dequeue only looks at available jobs so a partial index serves us well.
+Moreover, dequeue sorts jobs by attempt and scheduled_at, which matches this index.
+*/
+CREATE INDEX idx_queue_dequeue_partial ON job_queue(queue, attempt, scheduled_at) WHERE status = 'available' :: job_status;