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Re: Order by behaviour

From: Kresimir Tonkovic <z-el(dot)tonkovic(at)chipoteka(dot)hr>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Order by behaviour
Date: 2005-12-23 14:31:15
Message-ID: 43AC0A33.3060806@chipoteka.hr (view raw or flat)
Thread:
Lists: pgsql-performance
Carlos Benkendorf wrote:

> Hi,
>  
> We have more than 200 customers running 8.0.3 and two weeks ago 
> started migration project to 8.1.1.After the first migration to 8.1.1 
> we had to return back to 8.0.3 because some applications were not 
> working right.
>  
> Our user told me that records are not returning more in the correct 
> order, so I started logging and saw that the select clause wasn´t not 
> used with the ORDER BY clause. It seemed a simple problem to be solved.
>  
> I asked the programmers that they should add the ORDER BY clause if 
> they need the rows in a certain order and they told me they could not 
> do it because it will cost too much and the response time is bigger 
> than not using ORDER BY. I disagreed with them because there was an 
> index with the same order needed for the order by. Before starting a 
> figth we decided to explain analyze both select types and discover who 
> was right. For my surprise the sele ct with order by was really more 
> expensive than the select without the order by. I will not bet any 
> more...;-)
>  
> For some implementation reason in 8.0.3 the query is returning the 
> rows in the correct order even without the order by but in 8.1.1 
> probably the implementation changed and the rows are not returning in 
> the correct order.
>  
> We need the 8.1 for other reasons but this order by behavior stopped 
> the migration project.
>  
> Some friends of the list tried to help us and I did some configuration 
> changes like increased work_mem and changed the primary columns from 
> numeric types to smallint/integer/bigint but even so the runtime and 
> costs are far from the ones from the selects without the ORDER BY clause.
>  
> What I can not understand is why the planner is not using the same 
> retrieving method with the order by clause as without the order by 
> clause. All the rows are retriev ed in the correct order in both 
> methods but one is much cheaper (without order by) than the other 
> (with order by). Should not the planner choice that one?
>  
> Can someone explain me why the planner is not choosing the same method 
> used with the selects without the order by clause instead of using a 
> sort that is much more expensive?

Maybe your table in the old database is clustered on an index that 
covers all ordered columns? Then, a sequential fetch of all rows would 
probably return them ordered. But there still should be no guarantee for 
this because postgres might first return the rows that are already in 
memory.

Just making wild guesses.

-- 
Krešimir Tonković
Z-el d.o.o.
Industrijska cesta 28, 10360 Sesvete, Croatia
Tel: +385 1 2022 758
Fax: +385 1 2022 741
Web: www.chipoteka.hr
e-mail: z-el(dot)tonkovic(at)chipoteka(dot)hr




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