Glenn Maynard wrote:
This rewrite allows getting the top N scores. Unfortunately, this one
takes 950ms for the same data. With 1000000 scores, it takes 14800ms.
SELECT s.* FROM score s, game g
WHERE s.game_id = g.id AND
s.id IN (
SELECT s2.id FROM score s2 WHERE s2.game_id=g.id ORDER BY s2.score
DESC LIMIT 1
);
You don't really need the join with game here, simplifying this into:
SELECT s.* FROM score s
WHERE s.id IN (
SELECT s2.id FROM score s2 WHERE s2.game_id=s.game_id ORDER BY
s2.score
DESC LIMIT 1
);
I don't think it makes it any faster, though.
You can also do this in a very nice and clean fashion using the upcoming
PG 8.4 window functions:
SELECT * FROM (
SELECT s.*, rank() OVER (PARTITION BY s.game_id ORDER BY score DESC)
AS rank FROM score s
) AS sub WHERE rank <= 5;
but I'm not sure how much faster it is. At least here on my laptop it
does a full index scan on score, which may or may not be faster than
just picking the top N values for each game using the index.
This seems simple: for each game, search for the highest score, and
then scan the tree to get the next N-1 highest scores. The first
version does just that, but the second one is doing a seq scan over
score.
You can do that approach with a SQL function:
CREATE FUNCTION topnscores(game_id int , n int) RETURNS SETOF score
LANGUAGE SQL AS $$
SELECT * FROM score s WHERE s.game_id = $1 ORDER BY score DESC LIMIT $2
$$;
SELECT (sub.ts).id, (sub.ts).score, (sub.ts).game_id
FROM (SELECT topnscores(g.id, 5) ts FROM game g) sub;
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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