On 8th of October, Tom Lane committed patch: Support index-only scans using the visibility map to avoid heap fetches. When a btree index contains all columns required by the query, and the visibility map shows that all tuples on a target heap page ...

Yeah. This one patch alone is worth upgrading to 8.4. On 3rd of December Heikki Linnakangas committed his patch. Commit message: Introduce visibility map. The visibility map is a bitmap with one bit per heap page, where a set bit indicates that all ...