Why wouldn't it do an index (or, really, an index only) scan in this case
Well, it did do an index scan (and a bitmap scan is a pretty good solution here), but as to why no indexonly scan, there is probably not enough assurance that it won't have to hit the heap heavily anyway. Try doing a SET enable_bitmapscan=0; and re-run with EXPLAIN ANALYZE. If you see a large number of "Heap Fetches", that could be why. Vacuum the table and try again after doing SET enable_bitmapscan=1;