Re: [HACKERS] Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)

От: Jim C. Nasby
Тема: Re: [HACKERS] Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)
Дата: ,
Msg-id: 20050406222536.GL93835@decibel.org
(см: обсуждение, исходный текст)
Ответ на: Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)  (Tom Lane)
Ответы: Re: [HACKERS] Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)  (Tom Lane)
Список: pgsql-performance

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Plan for relatively simple query seems to be very inefficient  (Arjen van der Meijden, )
 Re: Plan for relatively simple query seems to be very inefficient  (Steve Atkins, )
  Re: Plan for relatively simple query seems to be very inefficient  (Arjen van der Meijden, )
 Re: Plan for relatively simple query seems to be very inefficient  (Tom Lane, )
  Re: Plan for relatively simple query seems to be very inefficient  (Arjen van der Meijden, )
   Re: Plan for relatively simple query seems to be very inefficient  (Tom Lane, )
    Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)  (Tom Lane, )
     Re: [HACKERS] Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)  ("Jim C. Nasby", )
      Re: [HACKERS] Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)  (Tom Lane, )
       Re: [HACKERS] Recognizing range constraints (was Re: Plan  (John A Meinel, )
        Re: [HACKERS] Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)  (Tom Lane, )
       Re: [HACKERS] Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)  ("Jim C. Nasby", )
     Re: Recognizing range constraints (was Re: Plan for  (Simon Riggs, )
     Re: Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)  (Bruno Wolff III, )
      Re: Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)  (Tom Lane, )
       Re: Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)  (Mischa, )
        Re: Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)  (Tom Lane, )
         Re: Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient)  (, )
 Re: Plan for relatively simple query seems to be very inefficient  ("Dave Held", )
  Re: Plan for relatively simple query seems to be very inefficient  (Tom Lane, )
  Re: Plan for relatively simple query seems to be very inefficient  (Tom Lane, )
 Re: Plan for relatively simple query seems to be very inefficient  (Mischa, )

On Wed, Apr 06, 2005 at 06:09:37PM -0400, Tom Lane wrote:
> Can anyone suggest a more general rule?  Do we need for example to
> consider whether the relation membership is the same in two clauses
> that might be opposite sides of a range restriction?  It seems like
>
>     a.x > b.y AND a.x < b.z

In a case like this, you could actually look at the  data in b and see
what the average range size is. If you wanted to get really fancy, the
optimizer could decide how best to access a based on each row of b.

> probably can be treated as a range restriction on a.x for this purpose,
> but I'm much less sure that the same is true of
>
>     a.x > b.y AND a.x < c.z

Well, this could end up being much trickier, since who knows how b and c
are related. Though thinking about it, although I threw out the
row-by-row analysis idea to be glib, that would actually work in this
case; you could take a look at what b and c look like each time 'through
the loop'.
--
Jim C. Nasby, Database Consultant               
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