The exact way to clear all these caches and to get cold cache performance again varies based on the OS. Just stopping the database server isn't enough, because the OS cache can be expected to still have plenty of information cached. On Linux, you can use the drop_caches feature to discard everything it has in its page cache. Here's a complete example of cleaning the data out of memory for this database on Linux:
$ pg_ctl stop $ sudo su - # sync # echo 3 > /proc/sys/vm/drop_caches # logout $ pg_ctl start -l $PGLOG
The sync here is to try to flush all data to the disk before we just blow away the caches. This drop_caches feature on Linux is not intended for regular production server use; it is more of a debugging feature ...
