Re: again on index usage

> [with the new effective_cache_size = 6400]

This seems way too low for a 512 Mb machine. Why does your OS
only use so little for filecache ? Is the rest used for processes ?
For the above number you need to consider OS cache and shared_buffers.
You can approximatly add them together minus a few %.

With the data you gave, a calculated value for effective_cache_size
would be 29370, assuming the random_page_cost is actually 4 on your
machine. 29370 might be a slight overestimate, since your new table
will probably still be somewhat sorted by date within one IP.

Try to measure IO/s during the seq scan and during the index path
and calculate the ratio. This should be done during an average workload
on the machine.

Andreas