Index "misbehavior" in PostgreSQL 8.2.2?

Salut,

I have four tables, two of which are relevant for this:

searchengine=# \d websites                                   Table "public.websites"  Column   |           Type
 |                       Modifiers                        
------------+--------------------------+-------------------------------------------------------id         | bigint
            | not null default nextval('websites_id_seq'::regclass)url        | text                     | not
nulltitle     | text                     | abstract   | text                     | lastindex  | timestamp with time
zone| authority  | bigint                   | not null default 0failed     | boolean                  | not null
defaultfalsespamminess | smallint                 | not null default 0 
Indexes:   "websites_pkey" PRIMARY KEY, btree (id)   "websites_url_key" UNIQUE, btree (url)   "websites_authority_key"
btree(authority)   "websites_failed_key" btree (failed)   "websites_lastindex_key" btree (lastindex)
"websites_spamminess_key"btree (spamminess) 

searchengine=# SELECT pg_size_pretty (pg_relation_size ('websites'));pg_size_pretty
----------------293 MB
(1 row)

searchengine=# SELECT COUNT(*) FROM websites;count
--------828778
(1 row)

searchengine=# \d sitewords   Table "public.sitewords"  Column   |  Type  | Modifiers
------------+--------+-----------id_site    | bigint | not nullid_keyword | bigint | not nullcount      | bigint | not
nullratio     | bigint |  
Indexes:   "sitewords_key" UNIQUE, btree (id_site, id_keyword)   "sitewords_id_keyword_key" btree (id_keyword)
"sitewords_id_site_key"btree (id_site) 

searchengine=# SELECT COUNT(*) FROM sitewords; count
----------46696260
(1 row)

searchengine=# SELECT pg_size_pretty (pg_relation_size ('sitewords'));pg_size_pretty
----------------3041 MB
(1 row)

searchengine=#

My effective_cache_size is set to 768MB.

I am trying to run a query:

searchengine=# EXPLAIN ANALYZE SELECT COUNT(me.id_keyword) * SUM(ratio) AS relevance, id_site FROM sitewords me JOIN
websitesws ON me.id_site = ws.id WHERE me.id_keyword IN (4241,28303) GROUP BY id_site ORDER BY relevance DESC LIMIT 10;
                                                                         QUERY PLAN
                                       

------------------------------------------------------------------------------------------------------------------------------------------------------------------Limit
(cost=150319.90..150319.91 rows=3 width=24) (actual time=9211.903..9212.013 rows=10 loops=1)  ->  Sort
(cost=150319.90..150319.91rows=3 width=24) (actual time=9211.895..9211.931 rows=10 loops=1)        Sort Key:
((count(me.id_keyword))::numeric* sum(me.ratio))        ->  HashAggregate  (cost=150319.81..150319.87 rows=3 width=24)
(actualtime=9180.744..9197.080 rows=3320 loops=1)              ->  Hash Join  (cost=60470.11..150116.47 rows=27112
width=24)(actual time=7816.860..9151.601 rows=5747 loops=1)                    Hash Cond: (me.id_site = ws.id)
         ->  Bitmap Heap Scan on sitewords me  (cost=609.31..85356.43 rows=27112 width=24) (actual time=3.245..137.026
rows=5748loops=1)                          Recheck Cond: (id_keyword = ANY ('{4241,28303}'::bigint[]))
       ->  Bitmap Index Scan on sitewords_id_keyword_key  (cost=0.00..602.53 rows=27112 width=0) (actual
time=2.169..2.169rows=5748 loops=1)                                Index Cond: (id_keyword = ANY
('{4241,28303}'::bigint[]))                   ->  Hash  (cost=45674.80..45674.80 rows=816080 width=8) (actual
time=7786.940..7786.940rows=829078 loops=1)                          ->  Seq Scan on websites ws  (cost=0.00..45674.80
rows=816080width=8) (actual time=0.026..4126.069 rows=829078 loops=1)Total runtime: 9212.679 ms 
(13 rows)

searchengine=#

The only way to get PostgreSQL to use the index appears to be to set the
random_page_cost to 1.0 (1.1 also doesn't work). In that case, I get:

searchengine=# EXPLAIN ANALYZE SELECT COUNT(me.id_keyword) * SUM(ratio) AS relevance, id_site FROM sitewords me JOIN
websitesws ON me.id_site = ws.id WHERE me.id_keyword IN (4241,28303) GROUP BY id_site ORDER BY relevance DESC LIMIT 10;
                                                                         QUERY PLAN
                                       

------------------------------------------------------------------------------------------------------------------------------------------------------------------Limit
(cost=78647.65..78647.66 rows=3 width=24) (actual time=418.368..418.478 rows=10 loops=1)  ->  Sort
(cost=78647.65..78647.66rows=3 width=24) (actual time=418.359..418.395 rows=10 loops=1)        Sort Key:
((count(me.id_keyword))::numeric* sum(me.ratio))        ->  HashAggregate  (cost=78647.57..78647.63 rows=3 width=24)
(actualtime=386.734..403.241 rows=3320 loops=1)              ->  Nested Loop  (cost=310.29..78444.23 rows=27112
width=24)(actual time=2.878..354.418 rows=5747 loops=1)                    ->  Bitmap Heap Scan on sitewords me
(cost=310.29..26849.19rows=27112 width=24) (actual time=2.789..135.915 rows=5748 loops=1)
RecheckCond: (id_keyword = ANY ('{4241,28303}'::bigint[]))                          ->  Bitmap Index Scan on
sitewords_id_keyword_key (cost=0.00..303.51 rows=27112 width=0) (actual time=1.762..1.762 rows=5748 loops=1)
                   Index Cond: (id_keyword = ANY ('{4241,28303}'::bigint[]))                    ->  Index Scan using
websites_pkeyon websites ws  (cost=0.00..1.89 rows=1 width=8) (actual time=0.017..0.022 rows=1 loops=5748)
           Index Cond: (me.id_site = ws.id)Total runtime: 418.791 ms 
(12 rows)

searchengine=#

which is fine. However, if I leave random_page_cost at 4 and do the following
query:

searchengine=# EXPLAIN ANALYZE select x.ratio * x.count, x.id_site from (SELECT me.id_site, count(me.id_site),
sum(me.ratio)as ratio FROM sitewords me WHERE me.id_keyword IN (4241, 28303) group by id_site) x join websites ws on
(ws.id= x.id_site);                                                                     QUERY PLAN
                                                

------------------------------------------------------------------------------------------------------------------------------------------------------Nested
Loop (cost=85559.77..85585.13 rows=3 width=48) (actual time=169.707..337.717 rows=3320 loops=1)  ->  HashAggregate
(cost=85559.77..85559.82rows=3 width=16) (actual time=169.567..184.975 rows=3321 loops=1)        ->  Bitmap Heap Scan
onsitewords me  (cost=609.31..85356.43 rows=27112 width=16) (actual time=2.897..137.590 rows=5748 loops=1)
RecheckCond: (id_keyword = ANY ('{4241,28303}'::bigint[]))              ->  Bitmap Index Scan on
sitewords_id_keyword_key (cost=0.00..602.53 rows=27112 width=0) (actual time=1.820..1.820 rows=5748 loops=1)
       Index Cond: (id_keyword = ANY ('{4241,28303}'::bigint[]))  ->  Index Scan using websites_pkey on websites ws
(cost=0.00..8.41rows=1 width=8) (actual time=0.025..0.029 rows=1 loops=3321)        Index Cond: (ws.id =
x.id_site)Totalruntime: 351.053 ms 
(9 rows)

searchengine=#

the result is even more acceptable than with the above JOIN statement.

The tables are analyzed, and I have vacuumed regularly.

Any idea why PostgreSQL attempts a full sequential scan here?

            Tonnerre