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Re: REPOST: Nested loops row estimates always too high

From: Ow Mun Heng <Ow(dot)Mun(dot)Heng(at)wdc(dot)com>
To: Carlo Stonebanks <stonec(dot)register(at)sympatico(dot)ca>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: REPOST: Nested loops row estimates always too high
Date: 2007-09-24 06:46:16
Message-ID: 1190616376.17050.51.camel@neuromancer.home.net (view raw or flat)
Thread:
Lists: pgsql-performance
On Thu, 2007-09-20 at 11:02 -0400, Carlo Stonebanks wrote:
> (SORRY FOR THE REPOST, I DON'T SEE MY ORIGINAL QUESTION OR ANY ANSWERS HERE)
> 
> I am noticing that my queries are spending a lot of time in nested loops. 
> The table/index row estimates are not bad, but the nested loops can be off 
> by a factor of 50. In any case, they are always too high.
> 
> Are the over-estimations below significant, and if so, is this an indication 
> of a general configuration problem?
Sounds much like the issue I was seeing as well.

> 
> Unique  (cost=67605.91..67653.18 rows=4727 width=16) (actual 
> time=8634.618..8637.918 rows=907 loops=1)

You can to rewrite the queries to individual queries to see it if helps.

In my case, I was doing

select a.a,b.b,c.c from 
(select a from x where) a <--- Put as a SRF
left join (
select b from y where ) b <--- Put as a SRF
on a.a = b.a




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Subject: [OT] Re: [Again] Postgres performance problem
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