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Re: What is syslog:duration reporting ... ?

From: Aldor <an(at)mediaroot(dot)de>
To: "Marc G(dot) Fournier" <scrappy(at)postgresql(dot)org>
Cc: pgsql-admin(at)postgresql(dot)org
Subject: Re: What is syslog:duration reporting ... ?
Date: 2005-08-25 18:50:25
Message-ID: 430E12F1.1080206@mediaroot.de (view raw or flat)
Thread:
Lists: pgsql-admin
As I know EXPLAIN ANALYZE runs the query. I think you are just running 
the query two times. The first time you run the query it will take a 
long time to be processed - after the first run the query planner will 
remember the best way to run the query so your second run runs much faster.

I can reproduce this behavior for some queries under 8.0.1 - so I'm not 
100% sure if it is the same behavior under 7.4.2.

I'm still wondering why you first query takes about 4107.987 ms - this 
kind of query has usually have to run much much faster.

When did you vacuumed the table the last time?

Marc G. Fournier wrote:
> 
> 'k, I've been wracking my brains over this today, and I'm either 
> mis-understanding what is being reported *or* its reporting wrong ...
> 
> According to syslog:
> 
> LOG:  duration: 4107.987 ms  statement: UPDATE session SET 
> hit_time=now() WHERE md5='8b8e7b7ff9b1b2ed5fc60218ced28d00';
> 
> But, if I do an EXPLAIN ANALYZE:
> 
> # explain analyze UPDATE session SET hit_time=now() WHERE 
> md5='702c6cb20d5eb254c3feb2991e7e5e31';
>                                                          QUERY PLAN
> ---------------------------------------------------------------------------------------------------------------------------- 
> 
>  Index Scan using session_md5_key on "session"  (cost=0.00..6.01 rows=1 
> width=93) (actual time=0.060..0.060 rows=0 loops=1)
>    Index Cond: (md5 = '702c6cb20d5eb254c3feb2991e7e5e31'::bpchar)
>  Total runtime: 0.171 ms
> (3 rows)
> 
> And it doesn't matter what value I put for md5, I still get <1ms ...
> 
> I could see some variations, but almost 4000x slower  to *really* run 
> the query vs an explain analyze?
> 
> This is with 7.4.2 ...
> 
> ----
> Marc G. Fournier           Hub.Org Networking Services (http://www.hub.org)
> Email: scrappy(at)hub(dot)org           Yahoo!: yscrappy              ICQ: 7615664
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 2: Don't 'kill -9' the postmaster
> 

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