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(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us