Re: ORDER BY ... LIMIT and JOIN

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On Sunday 09 August 2009 21:26:08 Fizu wrote:
>                ->  Index Scan using country_ranking_user_idx on "user"
>  (cost=0.00..4807.25 rows=1710 width=143) (actual
> time=20.923..4898.931 rows=1972 loops=1)
>                      Index Cond: (country_id = 5)

The statistics looks good now, but almost all the time is still spent on 
fetching users with country_id = 5. The actual ordering is only a tiny part of 
the full cost. Why it takes time probably depends on your hardware in relation 
to database size. I guess the database doesn't fit in ram? What settings have 
you changed?

Clustering users on country_ranking_user_idx would probably help for this 
specific case, but if it is a good idea depends on what other queries need to 
be fast. If the table or indexes are bloated then clustering on any index or 
doing reindex might do it.

/Michael

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