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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: pgsql-performance@postgresql.org
Subject: Re: Performance issues
Date: Tue, 17 Mar 2015 16:30:24 +0100
Message-ID: <55084890.6000907@2ndquadrant.com> (raw)
In-Reply-To: <me9guu$cvp$1@ger.gmane.org>
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On 17.3.2015 16:24, Thomas Kellerer wrote:
> Tomas Vondra schrieb am 17.03.2015 um 15:43:
>> On 17.3.2015 15:19, Thomas Kellerer wrote:
>>> Tomas Vondra schrieb am 17.03.2015 um 14:55:
>>>>  (2) using window functions, e.g. like this:
>>>>
>>>>      SELECT * FROM (
>>>>        SELECT *,
>>>>             ROW_NUMBER() OVER (PARTITION BY touchpoint_execution_id
>>>>                                ORDER BY FROM max_creation_dt) AS rn
>>>>        FROM s_f_touchpoint_execution_status_history
>>>>      ) foo WHERE rn = 1
>>>>
>>>>      But estimating this is also rather difficult ...
>>>
>>>
>>> From my experience rewriting something like the above using DISTINCT 
>>> ON is usually faster.
>>
>> How do you get the last record (with respect to a timestamp column)
>> using a DISTINCT ON?
> 
> You need to use "order by ... desc". See here: http://sqlfiddle.com/#!15/d4846/2

Nice, thanks!

> 
> Btw: your row_number() usage wouldn't return the "latest" row either. 
> It would return the "oldest" row.

Oh, right. I forgot the DESC in the window.


-- 
Tomas Vondra                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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