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Re: Same query doing slow then quick

From: Julien Cigar <jcigar(at)ulb(dot)ac(dot)be>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Same query doing slow then quick
Date: 2012-09-26 13:21:38
Message-ID: 50630162.2090603@ulb.ac.be (view raw or flat)
Thread:
Lists: pgsql-performance
On 09/26/2012 15:03, FFW_Rude wrote:
> Here is the answer to Ray Stell who send me the wiki page of Slow Query. I
> hope i detailed all you wanted (i basicly pasted the page and add my
> answers).
>
> Full Table and Index Schema:
>
> schema tables_adresses
> "Tables"
> tables_adresses.adresses_XX (id (serial), X(Double precision),Y (Double
> precision)).
> "Indexes"
> adresses_XX_pkey (Primary key, btree)
> calcul_XX (non unique, Btree on X,Y)
>
> schema tables_gps
> "Tables"
> tables_gps.gps_XX (id (int),x_max(numeric(10,5)), y_max
> (numeric(10,5)),x_min(numeric(10,5)),y_min(numeric(10,5)))
> "Indexes"
> calculs_XX (non unique Btree x_min,x_max,y_min,y_max)
> gps_10_pkey (Primary key on id btree)
>
> Approximate rows 250000.
> No large objects in it (just data)
> No NULL
> receives a large number of UPDATEs or DELETEs regularly
> is growing daily
>
> I can't post an EXPLAIN ANALYZE because of the 6hour query time.
>
> Postgres version: 9.1
>
> History: was this query always slow, : "YES"
>
> Hardware: Ubuntu server last version 32bits
>
> Daily VACUUM FULL ANALYZE, REINDEX TABLE on all the tables.
>
> WAL Configuration: Whats a WAL ?
>
> GUC Settings: i didn't change anything. All is standard.
>
> shared_buffers should be 10% to 25% of available RAM (it's on 24MB and can't
> go higher. The server has 4Gb)
>
> effective_cache_size should be 75% of available RAM =>  I don't now what this
> is.

before looking further, please configure shared_buffers and 
effective_cache_size properly, it's fundamental
you'll probably need to raise SHMALL/SHMMAX, take a look at: 
http://www.postgresql.org/docs/current/static/kernel-resources.html
for 4GB of RAM I start with shared_buffers to 512MB and 
effective_cache_size to 2GB

> Test changing work_mem: increase it to 8MB, 32MB, 256MB, 1GB. Does it make a
> difference? "No"

default work_mem is very small, set it to something like 16MB

>
>
>
> --
> View this message in context: http://postgresql.1045698.n5.nabble.com/Same-query-doing-slow-then-quick-tp5725486p5725491.html
> Sent from the PostgreSQL - performance mailing list archive at Nabble.com.
>
>


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