Re: [WIP] cache estimates, cache access cost

On 06/19/2011 09:38 AM, Greg Stark wrote:
> There's another problem which I haven't seen mentioned. Because the
> access method will affect the cache there's the possibility of
> feedback loops. e.g. A freshly loaded system prefers sequential scans
> for a given table because without the cache the seeks of random reads
> are too expensive...

Not sure if it's been mentioned in this thread yet, but he feedback 
issue has popped up in regards to this area plenty of times.  I think 
everyone who's producing regular input into this is aware of it, even if 
it's not mentioned regularly.  I'm not too concerned about the specific 
case you warned about because I don't see how sequential scan vs. index 
costing will be any different on a fresh system than it is now.  But 
there are plenty of cases like it to be mapped out here, and many are 
not solvable--they're just something that needs to be documented as a risk.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support  www.2ndQuadrant.us