Re: Cost of sort/order by not estimated by the query planner

The table is clustered by by blog_id.
So, for testing purpose, i tried an ORDER BY blog_id.

limit 500 :
-------------
explain analyze SELECT *
FROM   _article
WHERE (_article.bitfield && getbit(0))
ORDER BY _article.blog_id ASC
LIMIT 500;
Limit  (cost=66229.90..66231.15 rows=500 width=1099) (actual
time=9.368..9.580 rows=500 loops=1)  ->  Sort  (cost=66229.90..66273.25 rows=17341 width=1099) (actual
time=9.367..9.443 rows=500 loops=1)        Sort Key: blog_id        Sort Method:  top-N heapsort  Memory: 660kB
-> Bitmap Heap Scan on _article  (cost=138.67..65365.82
 
rows=17341 width=1099) (actual time=0.905..4.042 rows=6729 loops=1)              Recheck Cond: (bitfield && B'1'::bit
varying)             ->  Bitmap Index Scan on idx_article_bitfield
 
(cost=0.00..134.33 rows=17341 width=0) (actual time=0.772..0.772
rows=6729 loops=1)                    Index Cond: (bitfield && B'1'::bit varying)Total runtime: 9.824 ms

Limit 5 :
----------
explain analyze SELECT *
FROM   _article
WHERE (_article.bitfield && getbit(0))
ORDER BY _article.blog_id ASC
LIMIT 5;
Limit  (cost=0.00..1419.22 rows=5 width=1099) (actual
time=125076.420..280419.143 rows=5 loops=1)  ->  Index Scan using idx_article_blog_id on _article
(cost=0.00..4922126.37 rows=17341 width=1099) (actual
time=125076.419..280419.137 rows=5 loops=1)        Filter: (bitfield && B'1'::bit varying)Total runtime: 280419.241 ms


-- 
Laurent "ker2x" Laborde
Sysadmin & DBA at http://www.over-blog.com/