adding 'limit' leads to very slow query
От | Michael McFarland |
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Тема | adding 'limit' leads to very slow query |
Дата | |
Msg-id | opsnaj0hctsvs6tg@localhost.localdomain обсуждение исходный текст |
Ответы |
Re: adding 'limit' leads to very slow query
Re: adding 'limit' leads to very slow query |
Список | pgsql-performance |
I'm trying to understand why a particular query is slow, and it seems like the optimizer is choosing a strange plan. See this summary: * I have a large table, with an index on the primary key 'id' and on a field 'foo'. > select count(*) from foo; 1,000,000 > select count(*) from foo where bar = 41; 7 * This query happens very quickly. > explain select * from foo where barId = 412 order by id desc; Sort () Sort key= id -> Index scan using bar_index on foo () Index cond: barId = 412 But this query takes forever > explain select * from foo where barId = 412 order by id desc limit 25; Limit () -> Index scan backward using primarykey_index Filter: barID = 412 Could anyone shed some light on what might be happening here? - Michael -- Using Opera's revolutionary e-mail client: http://www.opera.com/mail/
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