Re: Questions on query planner, join types, and work_mem

Bruce Momjian wrote:
> Robert Haas wrote:
> > On Wed, Aug 11, 2010 at 9:42 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> > > This confused me. ?If we are assuing the data is in
> > > effective_cache_size, why are we adding sequential/random page cost to
> > > the query cost routines?
> >
> > See the comments for index_pages_fetched(). We basically assume that
> > all data starts uncached at the beginning of each query - in fact,
> > each plan node. effective_cache_size only measures the chances that
> > if we hit the same block again later in the execution of something
> > like a nested-loop-with-inner-indexscan, it'll still be in cache.
> >
> > It's an extremely weak knob, and unless you have tables or indices
> > that are larger than RAM, the only mistake you can make is setting it
> > too low.
>
> The attached patch documents that there is no assumption that data
> remains in the disk cache between queries. I thought this information
> might be helpful.

Applied.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +