Excerpts from Hans-Jürgen Schönig's message of mié ago 11 08:21:10 -0400 2010:

> same with limit ...
> test=# explain analyze select * from t_test order by x, y limit 20;

But if you put the limit in a subquery which is ordered by the
known-indexed condition, it is very fast:

alvherre=# explain analyze select * from (select * from t_test order by x limit 
20) f order by x, y;
                                                               QUERY PLAN       
 Sort  (cost=1.24..1.29 rows=20 width=8) (actual time=0.252..0.296 rows=20 
   Sort Key: t_test.x, t_test.y
   Sort Method:  quicksort  Memory: 26kB
   ->  Limit  (cost=0.00..0.61 rows=20 width=8) (actual time=0.051..0.181 
rows=20 loops=1)
         ->  Index Scan using idx_aaaaa on t_test  (cost=0.00..30408.36 
rows=1000000 width=8) (actual time=0.046..0.098 rows=20 loops=1)
 Total runtime: 0.425 ms
(6 filas)

I guess it boils down to being able to sort a smaller result set.

Álvaro Herrera <alvhe...@commandprompt.com>
The PostgreSQL Company - Command Prompt, Inc.
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