Re: Why is sorting on two columns so slower than sorting on one column?
От | Li Jie |
---|---|
Тема | Re: Why is sorting on two columns so slower than sorting on one column? |
Дата | |
Msg-id | 001801cba2ad$cf3b4100$2ad118ac@A0078508 обсуждение исходный текст |
Ответ на | Why is sorting on two columns so slower than sorting on one column? (Jie Li <jay23jack@gmail.com>) |
Список | pgsql-hackers |
Hi Marti, Thanks for your help! I guess I understand what you mean, a clustered index will make sorting as cheap as a seq scan, right? But what I meant is, is there any potential optimization for the backend implementation? Intuitively, if sorting on one columnor two columns will incur the same I/O costs, why should there be so much difference? Thanks, Li Jie ----- Original Message ----- From: "Marti Raudsepp" <marti@juffo.org> To: "Jie Li" <jay23jack@gmail.com> Cc: "pgsql-hackers" <pgsql-hackers@postgresql.org> Sent: Thursday, December 23, 2010 10:17 PM Subject: Re: [HACKERS] Why is sorting on two columns so slower than sorting on one column? On Thu, Dec 23, 2010 at 09:33, Jie Li <jay23jack@gmail.com> wrote: > While the first sorting takes > about only 6 seconds, the second one takes over 30 seconds, Is this too > much than expected? Is there any possible optimization ? If you're doing these queries often, you should: CREATE INDEX ix_big_wf_age_id ON big_wf (age, id) If that's still not fast enough, you can physically sort rows in the table using the newly created index: CLUSTER big_wf USING ix_big_wf_age_id; Please post back your results. :) Regards, Marti
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