Re: turn off caching for performance test

[Greg Smith <greg@xxxxxxxxxxxxxxx>]

Willy-Bas Loos wrote:

But that's only the OS cache. There's also the shared_buffers, which are a postgres specific thing.
I've found DISCARD in the manual, but that only influences a single session, not the shared buffers.

I reckon restarting the cluster should help, would it wipe out the cache? (pg_ctlcluster 8.3 main restart)
Or is there a more graceful way?

Stop the cluster; flush the OS cache; start the cluster again.  Now you have a clean cache to retest again.  No easier way that's reliable.  If you try to clear out the database by doing things like scanning large tables not involved in the query, you'll discover features in PostgreSQL will specifically defeat that from using more than a small portion of the cache.  Better to just do a full shutdown.

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@xxxxxxxxxxxxxxx   www.2ndQuadrant.us