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

From: Tom Lane
Subject: Re: Recognizing range constraints (was Re: [PERFORM] Plan for relatively simple query seems to be very inefficient)
Date: ,
Msg-id: 6594.1112826910@sss.pgh.pa.us
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In response to: Re: Recognizing range constraints (was Re: [PERFORM] Plan for relatively simple query seems to be very inefficient)  ("Jim C. Nasby")
Responses: Re: Recognizing range constraints (was Re: [PERFORM] Plan for relatively simple query seems to be very inefficient)  ("Jim C. Nasby")
Re: Recognizing range constraints (was Re: [PERFORM] Plan  (John A Meinel)
List: pgsql-hackers


"Jim C. Nasby" <> writes:
> 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.

Not with the current statistics --- you'd need some kind of cross-column
statistics involving both y and z.  (That is, I doubt it would be
helpful to estimate the average range width by taking the difference of
independently-calculated mean values of y and z ...)  But yeah, in
principle it would be possible to make a non-default estimate.

            regards, tom lane


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