Re: Performance issues

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

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