Message-ID: <54FB1CDA.1080606@2ndquadrant.com>
Date: Sat, 07 Mar 2015 16:44:26 +0100
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Organization: 2ndQuadrant
User-Agent: Mozilla/5.0 (X11; Linux x86_64;
 rv:31.0) Gecko/20100101 Thunderbird/31.5.0
MIME-Version: 1.0
To: Jeff Janes <jeff.janes@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>
CC: Kevin Grittner <kgrittn@ymail.com>,
 "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>,
 Gunnlaugur Thor Briem <gunnlaugur@gmail.com>
Subject: Re: EXPLAIN (no ANALYZE) taking an hour for INSERT FROM
 SELECT
References: <54F9E179.4000804@2ndquadrant.com>
	<1407502480.21180.1425691482543.JavaMail.yahoo@mail.yahoo.com>
	<27245.1425692294@sss.pgh.pa.us>
 <CAMkU=1zxAB4AQySxex_DXP2mt+Ab=S+nQw+2M1rY7CFJzgYfTQ@mail.gmail.com>
In-Reply-To: 
 <CAMkU=1zxAB4AQySxex_DXP2mt+Ab=S+nQw+2M1rY7CFJzgYfTQ@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org

On 7.3.2015 03:26, Jeff Janes wrote:
> On Fri, Mar 6, 2015 at 5:38 PM, Tom Lane <tgl@sss.pgh.pa.us
> <mailto:tgl@sss.pgh.pa.us>> wrote:
> 
> But the actual query is using a seq scan, and so it would hint the
> table in efficient sequential order, rather than hinting the table
> haphazardly in index order like probing the endpoint does.

I think this has nothing to do with the plan itself, but with the
estimation in optimizer - that looks up the range from the index in some
cases, and that may generate random I/O to the table.

> 
>> Also, it's less than clear why only this particular query is
>> showing any stress. Dead rows should be a hazard for anything,
>> especially if there are enough of them to require hours to re-hint.
>> And why wouldn't autovacuum get to them first?
> 
> 
> Say the timing of this query is such that 10% of the parent turns
> over between invocations of this query, and that this 10% is all at
> the end of some index but random over the table heap. If autovac
> kicks in at 20% turn over, then half the time autovac does get to
> them first, and half the time it doesn't. It would be interesting to
> know if this query is bad every time it is planner, or just
> sometimes.

Yeah, this might be the reason. Another possibility is that this is part
of some large batch, and autovacuum simply did not have change to do the
work.

regards

-- 
Tomas Vondra                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


-- 
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance