Re: Performance issues

Vivekanand Joshi <vjoshi@zetainteractive.com> writes:

> So, here is the first taste of success and which gives me the confidence
> that if properly worked out with a good hardware and proper tuning,
> PostgreSQL could be a good replacement.
>
> Out of the 9 reports which needs to be migrated in PostgreSQL, 3 are now
> running.
>
> Report 4 was giving an issue and I will see it tomorrow.
>
> Just to inform you guys that, the thing that helped most is setting
> enable_nestloops to false worked. Plans are now not miscalculated.
>
> But this is not a production-suitable setting. So what do you think how to
> get a work around this?

Consider just disabling that setting for 1 or a few odd queries you have
for which they are known  to plan badly.

begin;
set local enable_nestloops to false;
select ...;
commit/abort;

I'd say never make that sort of setting DB or cluster-wide.


>
>
> Regards,
> Vivek
>
> -----Original Message-----
> From: pgsql-performance-owner@postgresql.org
> [mailto:pgsql-performance-owner@postgresql.org] On Behalf Of Tomas Vondra
> Sent: Tuesday, March 17, 2015 9:00 PM
> To: pgsql-performance@postgresql.org
> Subject: Re: [PERFORM] Performance issues
>
> 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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