Re: Extracting superlatives - SQL design philosophy

Hi all,

Just for your information, and this is not related to PG directly:

Teradata provides a “qualify” syntax which works as a filtering condition on
a windowed function result. This is the only DB allowing this direct
filtering on windowed functions, from what I know.

So, as an example, the query you ask for becomes very easy on this database:

select

city, temp, date

from bar

qualify row_number() over (partition by city order by temp desc)=1

This is very practical indeed (you can mix it also with classical
where/having/group by syntaxes).

On postgres, you may get the same result using an inner query (sorry, I
can’t test it for now) such as:

select

city, temp, date

from

(select city, temp, date, row_number() over (partition by city order by temp
desc) as nr

from bar ) a1

where nr=1

Julien Theulier

De : pgsql-performance-owner(at)postgresql(dot)org
[mailto:pgsql-performance-owner(at)postgresql(dot)org] De la part de Mose
Envoyé : mercredi 24 février 2010 22:50
À : Dave Crooke
Cc : pgsql-performance
Objet : Re: [PERFORM] Extracting superlatives - SQL design philosophy

Can you try using window functions?

Something like this:

select distinct

city,

first_value(temp) over w as max_temp,

first_value(date) over w as max_temp_date

from

cities

window w as (partition by city order by temp desc)

<http://www.postgresql.org/docs/current/static/tutorial-window.html>
http://www.postgresql.org/docs/current/static/tutorial-window.html

- Mose

On Wed, Feb 24, 2010 at 1:31 PM, Dave Crooke < <mailto:dcrooke(at)gmail(dot)com>
dcrooke(at)gmail(dot)com> wrote:

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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