I've seen the same thing. I understand Postgres has addressed this issue in current versions but the version I was just doing contract work with (a year old I think) had memory leaks that meant it had to be restarted just about daily b/c the performance would spiral down to almost nothing. The MySQL I was working with 3 years ago had no trouble driving up to 10 million page views a day of data from a couple dozen different tables and I didn't admin it but I don't recall it needed restarting often if ever.