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

От: Hannu Krosing
Тема: Re: Questions on query planner, join types, and work_mem
Дата: ,
Msg-id: 1280951504.3735.93.camel@hvost
(см: обсуждение, исходный текст)
Ответ на: Re: Questions on query planner, join types, and work_mem  (Greg Smith)
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, ) 
On Wed, 2010-08-04 at 15:16 -0400, Greg Smith wrote:
> Hannu Krosing wrote:
> > There was ample space for keeping the indexes in linux cache (it has 1GB
> > cached currently) though the system may have decided to start writing it
> > to disk, so I suspect that most of the time was spent copying random
> > index pages back and forth between shared buffers and disk cache.
> >
>
> Low shared_buffers settings will result in the same pages more often
> being written multiple times per checkpoint,

Do you mean "written to disk", or written out from shared_buffers to
disk cache ?

>  particularly index pages,
> which is less efficient than keeping in the database cache and updating
> them there.  This is a slightly different issue than just the overhead
> of copying them back and forth; by keeping them in cache, you actually
> reduce writes to the OS cache.

That's what I meant. Both writes to and read from the OS cache take a
significant amount of time once you are not doing real disk I/O.

> What I do to quantify that is...well,
> the attached shows it better than I can describe; only works on 9.0 or
> later as it depends on a feature I added for this purpose there.  It
> measures exactly how much buffer cache churn happened during a test, in
> this case creating a pgbench database.
>
> --
> Greg Smith  2ndQuadrant US  Baltimore, MD
> PostgreSQL Training, Services and Support
>    www.2ndQuadrant.us
>
>

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