Re: Planner choosing NestedLoop, although it is slower...

From: Mario Splivalo
Subject: Re: Planner choosing NestedLoop, although it is slower...
Date: ,
Msg-id: 4E1CFD33.7040407@megafon.hr
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In response to: Re: Planner choosing NestedLoop, although it is slower...  (Mario Splivalo)
List: pgsql-performance


On 07/13/2011 02:53 AM, Mario Splivalo wrote:
> On 07/13/2011 12:39 AM, Tom Lane wrote:
>> Mario Splivalo<> writes:
>>> On 07/12/2011 10:04 PM, Tom Lane wrote:
>>>> What you need to look into is why the estimated join size is 9400 rows
>>>> when the actual join size is zero. Are both tables ANALYZEd? Are you
>>>> intentionally selecting rows that have no join partners?
>>
>>> Yes, both tables have been ANALYZEd. What do you mean, intentilnaly
>>> selecting rows taht have no join partners?
>>
>> I'm wondering why the actual join size is zero. That seems like a
>> rather unexpected case for a query like this.
>
> Yes, seems that planer gets confused by LIMIT. This query:
>
> select * from tubesite_object join tubesite_image on id=object_ptr_id
> where site_id = 8 and pub_date < '2011-07-12 13:25:00' order by pub_date
> desc ;
>
> Does not choose Nested Loop, and is done instantly (20 ms), and returns
> no rows. However, if I add LIMIT at the end, it chooses NestedLoop and
> it takes 500ms if I'm alone on the server, and 10+ seconds if there 50+
> connections on the server.

As explained/suggested by RhodiumToad on IRC, adding composite index on
(site_id, pub_date) made nestedloop query finish in around 100 seconds!

Thank you!

    Mario


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