This looks to be a perfect use for SELECT DISTINCT ON: SELECT DISTINCT ON (city) * FROM bar ORDER BY city, temp desc Or am I misunderstanding the issue? Garrett Murphy -----Original Message----- From: pgsql-performance-owner@xxxxxxxxxxxxxx [mailto:pgsql-performance-owner@xxxxxxxxxxxxxx] On Behalf Of Dave Crooke Sent: Wednesday, February 24, 2010 2:31 PM To: pgsql-performance Subject: 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) -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
