From: <gnuoytr@rcn.com>
Subject: Re: Postgres refusing to use >1
 core
To: pgsql-performance@postgresql.org
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Date: Wed, 11 May 2011 19:07:57 -0400 (EDT)

---- Original message ----
>Date: Wed, 11 May 2011 17:04:50 -0500
>From: pgsql-performance-owner@postgresql.org (on behalf of Shaun Thomas <sthomas@peak6.com>)
>Subject: Re: [PERFORM] Postgres refusing to use >1 core  
>To: <gnuoytr@rcn.com>
>Cc: Scott Marlowe <scott.marlowe@gmail.com>,Craig Ringer <craig@postnewspapers.com.au>,Aren Cambre <aren@arencambre.com>,<pgsql-performance@postgresql.org>
>
>On 05/11/2011 02:53 PM, gnuoytr@rcn.com wrote:
>
>> So, the $64 question:  how did you find an engagement where, to bend
>> Shakespeare, "first thing we do, is kill all the coders" isn't
>> required?
>
>It's just one of those things you have to explain. Not just how to fix 
>it, but *why* doing so fixes it. It's also not really a fair expectation 
>in a lot of ways. Even when a coder uses all SQL, their inexperience in 
>the engine can still ruin performance. We spend years getting to know 
>PostgreSQL, or just general DB techniques. They do the same with coding. 
>And unless they're a developer for a very graphics intensive project, 
>they're probably not well acquainted with set theory.
>
>Just today, I took a query like this:
>
>   UPDATE customer c
>      SET c.login_counter = a.counter
>     FROM (SELECT session_id, count(*) as counter
>             FROM session
>            WHERE date_created >= CURRENT_DATE
>            GROUP BY session_id) a
>    WHERE c.process_date = CURRENT_DATE
>      AND c.customer_id = a.session_id
>
>And suggested this instead:
>
>   CREATE TEMP TABLE tmp_login_counts AS
>   SELECT session_id, count(1) AS counter
>     FROM auth_token_arc
>    WHERE date_created >= CURRENT_DATE
>    GROUP BY session_id
>
>   UPDATE reporting.customer c
>      SET login_counter = a.counter
>     FROM tmp_login_counts a
>    WHERE c.process_date = CURRENT_DATE
>      AND c.customer_id = a.session_id
>
>The original query, with our very large tables, ran for over *two hours* 
>thanks to a nested loop iterating over the subquery. My replacement ran 
>in roughly 30 seconds. If we were using a newer version of PG, we could 
>have used a CTE. But do you get what I mean? Temp tables are a fairly 
>common technique, but how would a coder know about CTEs? They're pretty 
>new, even to *us*.
>
>We hold regular Lunch'n'Learns for our developers to teach them the 
>good/bad of what they're doing, and that helps significantly. Even hours 
>later, I see them using the techniques I showed them. The one I'm 
>presenting soon is entitled '10 Ways to Ruin Performance' and they're 
>all specific examples taken from day-to-day queries and jobs here, all 
>from different categories of mistake. It's just a part of being a good DBA.
>
>-- 
>Shaun Thomas
>OptionsHouse | 141 W. Jackson Blvd. | Suite 800 | Chicago IL, 60604
>312-676-8870
>sthomas@peak6.com
>
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>
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You're (both) fortunate to have Suits and colleagues who are open to doing this A Better Way.  Bless you.

Regards,
Robert