pg.ddx.io  pgsql-performance@postgresql.org mailing list archive  

help / color / mirror / Atom feed
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Robert Haas <robertmhaas@gmail.com>
To: hlinnaka <hlinnaka@iki.fi>
Cc: pgsql-hackers@postgresql.org <pgsql-hackers@postgresql.org>
Subject: Re: PATCH: adaptive ndistinct estimator v4
Date: Fri, 01 May 2015 03:24:14 +0200
Message-ID: <5542D5BE.4070109@2ndquadrant.com> (raw)
In-Reply-To: <CA+TgmoYzriVcAefToJNhwJoJH-c1na71YveLvwC637q2RL1wUA@mail.gmail.com>
References: <541A2335.3060100@agliodbs.com>
	<WM!ce2c941557a0e9090a3cb66d0469128c782bb4d7b1f60d15a5c91279578d4c6fd237efa5b4660e94a0056752c695113e!@asav-2.01.com>
	<542C4E60.3030505@agliodbs.com>
	<CA+U5nMKqqPNpSr2CQ2V0=9bjWVv-OA28o3BoxYP=7YVfyswdzw@mail.gmail.com>
	<CAEYLb_UTz=dtxYOCYF_D1s7wQJfoZUrey6Fjxq08VFueXWoEKQ@mail.gmail.com>
	<WM!341e0135c2f371f48830484cdb3da37506d776453b10db7a9af11d648214c4b437402f04e7ccb3d30918cbc64dfee95d!@asav-1.01.com>
	<542DADEB.2080103@agliodbs.com>
	<CAM-w4HNfZymQmTu3+TxQQD-e6_410-sDnZBaW8neurmTFh4GbA@mail.gmail.com>
	<f4c06746b31e468f9550fb9924798da3.squirrel@2.emaily.eu>
	<54381D20.9010501@fuzzy.cz>
	<CAMkU=1yoiQt6+_XMACL=PTv-aHwc1E0BgiHGhKW3HL+ByN6zpw@mail.gmail.com>
	<5472416C.3080506@fuzzy.cz>
	<5483B346.6000203@fuzzy.cz>
	<551AEF45.7010700@2ndquadrant.com>
	<CA+TgmoaCsdM5WSpaVNmbWBN8+wuOGfYt1reZttGmDG1CnJvtLQ@mail.gmail.com>
	<55429F44.2070205@iki.fi>
	<CA+TgmoYzriVcAefToJNhwJoJH-c1na71YveLvwC637q2RL1wUA@mail.gmail.com>
List-Unsubscribe: <mailto:majordomo@postgresql.org?body=unsub%20pgsql-hackers>



On 05/01/15 00:18, Robert Haas wrote:
> On Thu, Apr 30, 2015 at 5:31 PM, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>>
>> You can override the ndistinct estimate with ALTER TABLE. I think
>> that's enough for an escape hatch.
>
> I'm not saying that isn't nice to have, but I don't think it really
> helps much here. Setting the value manually requires that you know
> what value to set, and you might not. If, on some workloads, the old
> algorithm beats the new one reliably, you want to be able to
> actually go back to the old algorithm, not manually override every
> wrong decision it makes. A GUC for this is pretty cheap insurance.

IMHO this is exactly the same situation as with the current ndistinct 
estimator. If we find out we'd have to use this workaround more 
frequently than before, then clearly the new estimator is rubbish and 
should not be committed.

In other words, I agree with Heikki.


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


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



view thread (17+ messages)  latest in thread

Message-ID: <5542D5BE.4070109@2ndquadrant.com>
Permalink:  ../5542D5BE.4070109@2ndquadrant.com/
Also on:    postgresql.org/message-id/5542D5BE.4070109@2ndquadrant.com

 · 

reply
Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: pgsql-performance@postgresql.org
  Cc: tomas.vondra@2ndquadrant.com, robertmhaas@gmail.com, hlinnaka@iki.fi, pgsql-hackers@postgresql.org
  Subject: Re: PATCH: adaptive ndistinct estimator v4
  In-Reply-To: <5542D5BE.4070109@2ndquadrant.com>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox