Re: [PERFORM] bad performance

От: Jeremy Harris
Тема: Re: [PERFORM] bad performance
Дата: ,
Msg-id: 9d8bede6-ce73-50a1-a6bc-53b77d168168@wizmail.org
(см: обсуждение, исходный текст)
Ответ на: Re: [PERFORM] bad performance  (Tom Lane)
Список: pgsql-performance

On 17/12/16 23:04, Tom Lane wrote:
> so that you don't need to use
> SELECT DISTINCT?  The sort/unique steps needed to do DISTINCT are
> eating a large part of the runtime,

Does a hash join result in a set of buckets that are then read out
in order?  It might, unless the sort method takes advantage of
partially-sorted inout, be cheaper (by log(num-buckets)) to sort/uniq
each bucket separately (and it would parallelize, too).
--
Cheers,
  Jeremy




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