Postgres 8.3 vs. 8.4 - Query plans and performance

Hello,

we have performance problems running several queries pon postgres 8.4 .
Using the previous version (8.3) our queries performs well
(The queries are quite complex, consisting of several sub-queries and
various spatial functions).

Using a new server with debian squeeze and postgres 8.4 from the
aptitude packages our queries won't finish and uses a lot of CPU time
(the used CPU runs mainly at 100%).

Indizes and datasets of the two databases are the same.
Regarding to the query plans there are some differences between the two
versions (using the same queries and after running VACUUM).

Are there some major changes from 8.3 to 8.4 that cause this bad
performance?


Regards,

Jo