Re: Sorted group by

Excerpts from Matthew Wakeling's message of mar ago 10 11:40:16 -0400 2010:

> I am trying to retrieve, for many sets of rows grouped on a couple of
> fields, the value of an ungrouped field where the row has the highest
> value in another ungrouped field.

I think this does what you want (schema is from the tenk1 table in the
regression database):

select string4 as group,
(array_agg(stringu1 order by unique1 desc))[1] as value
from tenk1
group by 1 ;

Please let me know how it performs with your data. The plan is rather simple:

regression=# explain analyze select string4 as group, (array_agg(stringu1 order by unique1 desc))[1] as value from tenk1 group by 1 ;
QUERY PLAN
───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
GroupAggregate (cost=0.00..1685.16 rows=4 width=132) (actual time=22.825..88.922 rows=4 loops=1)
-> Index Scan using ts4 on tenk1 (cost=0.00..1635.11 rows=10000 width=132) (actual time=0.135..33.188 rows=10000 loops=1)
Total runtime: 89.348 ms
(3 filas)

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