Re: "micro bucket sort" ...

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 loops=1)  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..0.098rows=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 <alvherre@commandprompt.com>
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