More info:
I upgraded the database from 8.4 to 9.1 using pg_upgrade, so I have no
way of running explain using 8.4.
I don't want to do an EXPLAIN ANALYZE because it would bog down the
server for too long. I know what it is doing, it's doing a seqscan.
This is a table with ~ 5.5 million rows and is ~100G in size. There are
4 indexes on this table, including one that matches what this query
needs exactly. Pg finds this index, but goes with the other alternative
(2), even though it thinks the index alternative (1) will be faster. I
don't even know what that means. I've never seen an EXPLAIN like that
before the 9.1 upgrade. I searched for "alternative" in the docs but
didn't find anything.