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 cant 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-ow...@postgresql.org [mailto:pgsql-performance-ow...@postgresql.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:dcro...@gmail.com> dcro...@gmail.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) -- Sent via pgsql-performance mailing list ( <mailto:pgsql-performance@postgresql.org> pgsql-performance@postgresql.org) To make changes to your subscription: <http://www.postgresql.org/mailpref/pgsql-performance> http://www.postgresql.org/mailpref/pgsql-performance
