Re: Why does the number of rows are different in actual and estimated.

От: Andrew Dunstan
Тема: Re: Why does the number of rows are different in actual and estimated.
Дата: ,
Msg-id: 50CA586A.30008@dunslane.net
(см: обсуждение, исходный текст)
Ответ на: Why does the number of rows are different in actual and estimated.  (AI Rumman)
Ответы: Re: Why does the number of rows are different in actual and estimated.  (Evgeny Shishkin)
Re: Why does the number of rows are different in actual and estimated.  (Claudio Freire)
Список: pgsql-performance

Скрыть дерево обсуждения

Why does the number of rows are different in actual and estimated.  (AI Rumman, )
 Re: Why does the number of rows are different in actual and estimated.  (Andrew Dunstan, )
  Re: Why does the number of rows are different in actual and estimated.  (Evgeny Shishkin, )
  Re: Why does the number of rows are different in actual and estimated.  (Claudio Freire, )
   Re: Why does the number of rows are different in actual and estimated.  (Andrew Dunstan, )
    Re: Why does the number of rows are different in actual and estimated.  (Evgeny Shishkin, )
     Re: Why does the number of rows are different in actual and estimated.  (Tom Lane, )
      Re: Why does the number of rows are different in actual and estimated.  (Evgeny Shishkin, )
       Re: Why does the number of rows are different in actual and estimated.  (Claudio Freire, )
        Re: Why does the number of rows are different in actual and estimated.  (AI Rumman, )
         Re: Why does the number of rows are different in actual and estimated.  (Claudio Freire, )

On 12/13/2012 05:12 PM, AI Rumman wrote:
> Why does the number of rows are different in actual and estimated?
>


Isn't that in the nature of estimates? An estimate is a heuristic guess
at the number of rows it will find for the given query or part of a
query. It's not uncommon for estimates to be out by several orders of
magnitude. Guaranteeing estimates within bounded accuracy and in a given
short amount of time (you don't want your planning time to overwhelm
your execution time) isn't possible.

cheers

andrew




В списке pgsql-performance по дате сообщения:

От: "Kevin Grittner"
Дата:
Сообщение: Re: hash join vs nested loop join
От: Huan Ruan
Дата:
Сообщение: Re: hash join vs nested loop join