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

От: Bruce Momjian
Тема: Re: Questions on query planner, join types, and work_mem
Дата: ,
Msg-id: 201102012024.p11KO6U20810@momjian.us
(см: обсуждение, исходный текст)
Ответ на: Re: Questions on query planner, join types, and work_mem  (Bruce Momjian)
Список: pgsql-performance

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Questions on query planner, join types, and work_mem  (Peter Hussey, )
 Re: Questions on query planner, join types, and work_mem  (Andres Freund, )
 Re: Questions on query planner, join types, and work_mem  (Tom Lane, )
  Re: Questions on query planner, join types, and work_mem  (Alvaro Herrera, )
   Re: Questions on query planner, join types, and work_mem  (Tom Lane, )
 Re: Questions on query planner, join types, and work_mem  (Jayadevan M, )
 Re: Questions on query planner, join types, and work_mem  (Tom Lane, )
  Re: Questions on query planner, join types, and work_mem  (Peter Hussey, )
   Re: Questions on query planner, join types, and work_mem  (Robert Haas, )
   Re: Questions on query planner, join types, and work_mem  (Tom Lane, )
   Re: Questions on query planner, join types, and work_mem  (Hannu Krosing, )
    Re: Questions on query planner, join types, and work_mem  (Robert Haas, )
     Re: Questions on query planner, join types, and work_mem  (Hannu Krosing, )
      Re: Questions on query planner, join types, and work_mem  (Tom Lane, )
       Re: Questions on query planner, join types, and work_mem  (Hannu Krosing, )
        Re: Questions on query planner, join types, and work_mem  (Hannu Krosing, )
         Re: Questions on query planner, join types, and work_mem  (Greg Smith, )
          Re: Questions on query planner, join types, and work_mem  (Greg Smith, )
          Re: Questions on query planner, join types, and work_mem  ("Kevin Grittner", )
           Re: Questions on query planner, join types, and work_mem  (Greg Smith, )
            Re: Questions on query planner, join types, and work_mem  ("Kevin Grittner", )
          Re: Questions on query planner, join types, and work_mem  (Hannu Krosing, )
         Re: Questions on query planner, join types, and work_mem  (Tom Lane, )
         Re: Questions on query planner, join types, and work_mem  (Hannu Krosing, )
          Re: Questions on query planner, join types, and work_mem  ("Kevin Grittner", )
         Re: Questions on query planner, join types, and work_mem  (Hannu Krosing, )
       Re: Questions on query planner, join types, and work_mem  (Bruce Momjian, )
        Re: Questions on query planner, join types, and work_mem  (Robert Haas, )
         Re: Questions on query planner, join types, and work_mem  (Bruce Momjian, )
          Re: Questions on query planner, join types, and work_mem  (Bruce Momjian, )
   Re: Questions on query planner, join types, and work_mem  ("Kevin Grittner", )
 Re: Questions on query planner, join types, and work_mem  (Greg Smith, )

Bruce Momjian wrote:
> Robert Haas wrote:
> > On Wed, Aug 11, 2010 at 9:42 PM, Bruce Momjian <> 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  <>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

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


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