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/