8.4

• The free space map used to track space that used to belong to now deleted data is tracked on disk instead of in shared memory, which removes any limits on how large it can be. This removes the need to worry about the max_fsm_pages and max_fsm_relations parameters, which are removed in this version. It also makes it much less likely that systems that are not being vacuumed frequently enough will end up in a situation where it's difficult to recover from that situation.
• The default value of the default_statistics_target parameter was increased from 10 to 100. This makes query planning time longer, and is responsible for a significant portion of the sometimes measured degradation in performance between 8.4 and 8.3 seen in many benchmarks. This mainly impacts trivial queries however. In more real-world ones, having improved statistics to work with will improve performance by making it less likely the query planner will pick a particularly bad execution plan. It's always possible...