So some of my output from an explain analyze here has a line that says this:
ex Scan using warranty_order_item_warranty_order_id_idx on warranty_order_item woi_1 (cost=0.57..277.53 rows=6 width=137) (actual time=0.110..0.111 rows=0 loops=1,010,844)
Not my strong suit but, I'm pretty sure that reads: "The index was queried 1M+ times and none of those inqueries resulted in a record being found". IIUC I'd be wondering why some form of hash join wasn't used...