On May 14, 2008, at 6:18 PM, Greg Smith wrote:
Also, how long should pages stay in the cache? (Assuming I have way morememory than the total size of all the tables/indexes.) Is there anytime-based expiration (in addition to LRU-based, which in my case shouldnever be resorted to)?Forever, at least as far as the PostgreSQL one is concerned. They only get evicted when a new buffer needs to be allocated and there's none available. It's not quite a LRU mechanism either. If you visit http://www.westnet.com/~gsmith/content/postgresql/ there's a presentation called "Inside the PostgreSQL Buffer Cache" that goes over how that part works. You'd probably find a look inside your system using pg_buffercache informative. Some of the sample queries I give only work on 8.3, but the "Buffer contents summary, with percentages" one should be OK on 8.1.
Hrm... don't seqscans use a separate set of buffers in 8.3? While technically those won't be evicted until needed, you're unlikely to find stuff hanging around there for terribly long...
-- Decibel!, aka Jim C. Nasby, Database Architect decibel@xxxxxxxxxxx Give your computer some brain candy! www.distributed.net Team #1828
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