From: "George Sexton" <georges@mhsoftware.com>
To: <pgsql-performance@postgresql.org>
References: <ca24673e1002241331l4099482aqc73649d93611767a@mail.gmail.com>
	<005a01cab59c$676fdb70$364f9250$@com>
In-Reply-To: <005a01cab59c$676fdb70$364f9250$@com>
Subject: Re: Extracting superlatives - SQL design philosophy
Date: Wed, 24 Feb 2010 15:00:09 -0700
Message-ID: <005d01cab59c$bbdeb1e0$339c15a0$@com>
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I missed something:

select=20
	B.City,
	MaxCityTemp.Temp,
	min(B.Date) as FirstMaxDate
from bar b=20
	INNER JOIN (select city,max(temp) as Temp from Bar group by City) as
MaxCityTemp
	ON B.City=3DMaxCityTemp.City AND B.Temp=3DMaxCityTemp.Temp
Group by
	B.City,
	MaxCityTemp.Temp

George Sexton
MH Software, Inc.
http://www.mhsoftware.com/
Voice: 303 438 9585
=20

> -----Original Message-----
> From: pgsql-performance-owner@postgresql.org =
[mailto:pgsql-performance-
> owner@postgresql.org] On Behalf Of George Sexton
> Sent: Wednesday, February 24, 2010 2:58 PM
> To: pgsql-performance@postgresql.org
> Subject: Re: [PERFORM] Extracting superlatives - SQL design philosophy
>=20
> You could do:
>=20
> select
> 	B.City,
> 	MaxCityTemp.Temp,
> 	min(B.Date) as FirstMaxDate
> from bar b
> 	INNER JOIN (select city,max(temp) as Temp from Bar group by City)
> as
> MaxCityTemp
> 	ON B.City=3DMaxCityTemp.City
> Group by
> 	B.City,
> 	MaxCityTemp.Temp
>=20
> George Sexton
> MH Software, Inc.
> http://www.mhsoftware.com/
> Voice: 303 438 9585
>=20
>=20
> > -----Original Message-----
> > From: pgsql-performance-owner@postgresql.org [mailto:pgsql-
> performance-
> > owner@postgresql.org] On Behalf Of Dave Crooke
> > Sent: Wednesday, February 24, 2010 2:31 PM
> > To: pgsql-performance
> > Subject: [PERFORM] Extracting superlatives - SQL design philosophy
> >
> > This is a generic SQL issue and not PG specific, but I'd like to get
> > an opinion from this list.
> >
> > Consider the following data:
> >
> > # \d bar
> > =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 Table "public.bar"
> > =A0Column |=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 =
Type=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 | Modifiers
> > --------+-----------------------------+-----------
> > =A0city=A0=A0 | character varying(255)=A0=A0=A0=A0=A0 |
> > =A0temp=A0=A0 | =
integer=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 |
> > =A0date=A0=A0 | timestamp without time zone |
> >
> > # select * from bar order by city, date;
> >    city    | temp |        date
> > -----------+------+---------------------
> >  Austin    |   75 | 2010-02-21 15:00:00
> >  Austin    |   35 | 2010-02-23 15:00:00
> >  Edinburgh |   42 | 2010-02-23 15:00:00
> >  New York  |   56 | 2010-02-23 15:00:00
> >  New York  |   78 | 2010-06-23 15:00:00
> > (5 rows)
> >
> > If you want the highest recorded temperature for a city, that's easy
> > to do, since the selection criteria works on the same column that we
> > are extracing:
> >
> > # select city, max(temp) from bar group by city order by 1;
> >    city    | max
> > -----------+-----
> >  Austin    |  75
> >  Edinburgh |  42
> >  New York  |  78
> > (3 rows)
> >
> >
> > However there is (AFAIK) no simple way in plain SQL to write a query
> > that performs such an aggregation where the aggregation criteria is
> on
> > one column and you want to return another, e.g. adding the the *date
> > of* that highest temperature to the output above, or doing a query =
to
> > get the most recent temperature reading for each city.
> >
> > What I'd like to do is something like the below (and I'm inventing
> > mock syntax here, the following is not valid SQL):
> >
> > -- Ugly implicit syntax but no worse than an Oracle outer join ;-)
> > select city, temp, date from bar where date=3Dmax(date) group by =
city,
> > temp order by city;
> >
> > or perhaps
> >
> > -- More explicit
> > select aggregate_using(max(date), city, temp, date) from bar group =
by
> > city, temp order by city;
> >
> > Both of the above, if they existed, would be a single data access
> > followed by and sort-merge.
> >
> > The only way I know how to do it involves doing two accesses to the
> > data, e.g.
> >
> > # select city, temp, date from bar a where date=3D(select =
max(b.date)
> > from bar b where a.city=3Db.city) order by 1;
> >    city    | temp |        date
> > -----------+------+---------------------
> >  Austin    |   35 | 2010-02-23 15:00:00
> >  Edinburgh |   42 | 2010-02-23 15:00:00
> >  New York  |   78 | 2010-06-23 15:00:00
> > (3 rows)
> >
> >
> > # explain select * from bar a where date=3D(select max(b.date) from =
bar
> > b where a.city=3Db.city) order by 1;
> >                                 QUERY PLAN
> > =
---------------------------------------------------------------------
> --
> > ---
> >  Sort  (cost=3D1658.86..1658.87 rows=3D1 width=3D528)
> >    Sort Key: a.city
> >    ->  Seq Scan on bar a  (cost=3D0.00..1658.85 rows=3D1 =
width=3D528)
> >          Filter: (date =3D (subplan))
> >          SubPlan
> >            ->  Aggregate  (cost=3D11.76..11.77 rows=3D1 width=3D8)
> >                  ->  Seq Scan on bar b  (cost=3D0.00..11.75 rows=3D1
> > width=3D8)     -- would be an index lookup in a real scenario
> >                        Filter: (($0)::text =3D (city)::text)
> > (8 rows)
> >
> > --
> > Sent via pgsql-performance mailing list (pgsql-
> > performance@postgresql.org)
> > To make changes to your subscription:
> > http://www.postgresql.org/mailpref/pgsql-performance
>=20
>=20
>=20
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> performance@postgresql.org)
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