Message-ID: <5500817C.3030609@2ndquadrant.com>
Date: Wed, 11 Mar 2015 18:55:08 +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>
CC: Tom Lane <tgl@sss.pgh.pa.us>, 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>
	<54FB1CDA.1080606@2ndquadrant.com>
 <CAMkU=1ywKaeECzjVpJUWEYPkA1AFGqL37tQxQ2j_pdXfkp8RrA@mail.gmail.com>
In-Reply-To: 
 <CAMkU=1ywKaeECzjVpJUWEYPkA1AFGqL37tQxQ2j_pdXfkp8RrA@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Precedence: bulk
Sender: pgsql-performance-owner@postgresql.org

On 11.3.2015 18:30, Jeff Janes wrote:
> On Sat, Mar 7, 2015 at 7:44 AM, Tomas Vondra
> <tomas.vondra@2ndquadrant.com <mailto:tomas.vondra@2ndquadrant.com>> wrote:
> 
>     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>
>     > <mailto: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.
> 
> 
> Right.  Tom was saying that the work needs to be done anyway, but it is
> just that some ways of doing the work are far more efficient than
> others.  It just happens that the executed plan in this case would do it
> more efficiently, (but in general you aren't going to get any less
> efficient than having the planner do it in index order).

Oh! Now I see what you meant. I parsed is as if you're suggesting that
the theory does not match the symptoms because the plan contains
sequential scan yet there's a lot of random I/O. But now I see that's
not what you claimed, so sorry for the noise.


-- 
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