Re: DB page cache/query performance

On May 14, 2008, at 6:18 PM, Greg Smith wrote:
>> 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)?
>
> 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...
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Decibel!, aka Jim C. Nasby, Database Architect  decibel@decibel.org
Give your computer some brain candy! www.distributed.net Team #1828