Re: Extracting superlatives - SQL design philosophy

I missed something:

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=MaxCityTemp.City AND B.Temp=MaxCityTemp.Temp
Group by
B.City,
MaxCityTemp.Temp

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

> -----Original Message-----
> From: pgsql-performance-owner(at)postgresql(dot)org [mailto:pgsql-performance-
> owner(at)postgresql(dot)org] On Behalf Of George Sexton
> Sent: Wednesday, February 24, 2010 2:58 PM
> To: pgsql-performance(at)postgresql(dot)org
> Subject: Re: [PERFORM] Extracting superlatives - SQL design philosophy
>
> You could do:
>
> 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=MaxCityTemp.City
> Group by
> B.City,
> MaxCityTemp.Temp
>
> George Sexton
> MH Software, Inc.
> http://www.mhsoftware.com/
> Voice: 303 438 9585
>
>
> > -----Original Message-----
> > From: pgsql-performance-owner(at)postgresql(dot)org [mailto:pgsql-
> performance-
> > owner(at)postgresql(dot)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
> >                 Table "public.bar"
> >  Column |            Type             | Modifiers
> > --------+-----------------------------+-----------
> >  city   | character varying(255)      |
> >  temp   | integer                     |
> >  date   | 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=max(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=(select max(b.date)
> > from bar b where a.city=b.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=(select max(b.date) from bar
> > b where a.city=b.city) order by 1;
> > QUERY PLAN
> > ---------------------------------------------------------------------
> --
> > ---
> > Sort (cost=1658.86..1658.87 rows=1 width=528)
> > Sort Key: a.city
> > -> Seq Scan on bar a (cost=0.00..1658.85 rows=1 width=528)
> > Filter: (date = (subplan))
> > SubPlan
> > -> Aggregate (cost=11.76..11.77 rows=1 width=8)
> > -> Seq Scan on bar b (cost=0.00..11.75 rows=1
> > width=8) -- would be an index lookup in a real scenario
> > Filter: (($0)::text = (city)::text)
> > (8 rows)
> >
> > --
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> > performance(at)postgresql(dot)org)
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>
>
>
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