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

help / color / mirror / Atom feed
From: Simon Riggs <simon@2ndQuadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Gregory Stark <stark@enterprisedb.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: jd@commandprompt.com <jd@commandprompt.com>
Cc: Josh Berkus <josh@agliodbs.com>
Cc: Greg Smith <gsmith@gregsmith.com>
Cc: pgsql-hackers@postgresql.org
Subject: Re: benchmarking the query planner
Date: Fri, 12 Dec 2008 18:31:57 +0000
Message-ID: <1229106717.8673.88.camel@ebony.2ndQuadrant> (raw)
In-Reply-To: <8020.1229105929@sss.pgh.pa.us>
References: <603c8f070812061019r5aca3aajeaa5493b11c5feff@mail.gmail.com>
	<603c8f070812092025k6325e0fcud001b205a507a634@mail.gmail.com>
	<4313.1228931932@sss.pgh.pa.us>
	<603c8f070812101828p1bd7df49yb8c916adedb49fe1@mail.gmail.com>
	<9565.1229018968@sss.pgh.pa.us>
	<1229033073.13078.179.camel@hp_dx2400_1>
	<87ej0etkfh.fsf@oxford.xeocode.com>
	<1229039044.13078.193.camel@hp_dx2400_1>
	<4136ffa0812111823u645b6ec9wdca60b3da4b00499@mail.gmail.com>
	<1229072675.13078.196.camel@hp_dx2400_1>
	<603c8f070812120344m67ef2c1fs41806cfb4ff9e396@mail.gmail.com>
	<1229093975.13078.275.camel@ebony.2ndQuadrant>
	<87tz99s7bu.fsf@oxford.xeocode.com>
	<1229100057.8673.29.camel@ebony.2ndQuadrant>
	<8020.1229105929@sss.pgh.pa.us>


On Fri, 2008-12-12 at 13:18 -0500, Tom Lane wrote:

> I seem to recall Greg suggesting that there were ways to estimate
> ndistinct without sorting, but short of a fundamental algorithm change
> there's not going to be a win here.

Hash table? Haas Stokes suggests a Bloom filter.

Why not keep the random algorithm we have now, but scan the block into a
separate hash table for ndistinct estimation. That way we keep the
correct random rows for other purposes.

> > Right now we may as well use a random number generator.
> 
> Could we skip the hyperbole please?

Some of the ndistinct values are very badly off, and in the common cases
I cited previously, consistently so.

Once I'm certain the rescue helicopter has seen me, I'll stop waving my
arms. (But yes, OK).

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support




view thread (89+ messages)  latest in thread

Message-ID: <1229106717.8673.88.camel@ebony.2ndQuadrant>
Permalink:  ../1229106717.8673.88.camel@ebony.2ndQuadrant/
Also on:    postgresql.org/message-id/1229106717.8673.88.camel@ebony.2ndQuadrant

 · 

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-hackers@postgresql.org
  Cc: simon@2ndQuadrant.com, tgl@sss.pgh.pa.us, stark@enterprisedb.com, robertmhaas@gmail.com, Kevin.Grittner@wicourts.gov, jd@commandprompt.com, josh@agliodbs.com, gsmith@gregsmith.com
  Subject: Re: benchmarking the query planner
  In-Reply-To: <1229106717.8673.88.camel@ebony.2ndQuadrant>

* 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