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

От: Alvaro Herrera
Тема: Re: Questions on query planner, join types, and work_mem
Дата: ,
Msg-id: 1280289550-sup-6155@alvh.no-ip.org
(см: обсуждение, исходный текст)
Ответ на: Re: Questions on query planner, join types, and work_mem  (Tom Lane)
Ответы: Re: Questions on query planner, join types, and work_mem  (Tom Lane)
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, )

Excerpts from Tom Lane's message of mar jul 27 20:05:02 -0400 2010:
> Peter Hussey <> writes:

> > 2) How is work_mem used by a query execution?
>
> Well, the issue you're hitting is that the executor is dividing the
> query into batches to keep the size of the in-memory hash table below
> work_mem.  The planner should expect that and estimate the cost of
> the hash technique appropriately, but seemingly it's failing to do so.
> Since you didn't provide EXPLAIN ANALYZE output, though, it's hard
> to be sure.

Hmm, I wasn't aware that hash joins worked this way wrt work_mem.  Is
this visible in the explain output?  If it's something subtle (like an
increased total cost), may I suggest that it'd be a good idea to make it
explicit somehow in the machine-readable outputs?


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