I am on PG 8.1.10 on RedHat Linux. Some page cache/query performance questions: Is there a way to tell whether a query is satisfied from memory cache or from disk. The only way I know of is based on the time the query takes (reported by EXPLAIN ANALYZE) -- comparing the first-time performance (after dropping the Linux page caches "echo 3 > /proc/sys/vm/drop_caches" and restarting the server) vs. subsequent invocations. Can I calculate exactly (based on PG config and OS parameters) the amount of memory available for DB page caches? Can someone walk me through the calculation or point me to a "for dummies" version. Also, how long should pages stay in the cache? (Assuming I have way more memory than the total size of all the tables/indexes.) Is there any time-based expiration (in addition to LRU-based, which in my case should never be resorted to)? All of this is prompted by watching the performance of some queries that according to my understanding should be served from page cache always (except for the first time after the server starts) -- instead they periodically exhibit execution times that look like they are not coming from cache. TIA, George
