Date: Sat, 1 Mar 2014 20:08:39 -0800 (PST)
From: David Johnston <polobo@yahoo.com>
To: pgsql-sql@postgresql.org
Message-ID: <1393733319509-5794267.post@n5.nabble.com>
In-Reply-To: <2042881838.607913.1393690147570.JavaMail.root@mobiletech.sk>
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Subject: Re: how to effectively SELECT new "customers"
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Jan Ostrochovsky wrote
>> Without incorporating additional meta-data about the purchases onto
>> the
>> customer table the most basic solution would be:
> 
>> SELECT DISTINCT customer_id FROM products WHERE date > (now() - '12
>> months'::interval)
>> EXCEPT
>> SELECT DISTINCT customer_id FROM products WHERE date <= (now() - '12
>> months'::interval)
> 
>> ---
> 
>> Another solution:
>> WHERE ... >12 AND NOT EXISTS (SELECT ... WHERE <= 12)
> 
>> David J.
> 
> subsidiary matter: in what circumstances is better to use EXCEPT and in
> what NOT EXISTS? 
> 
> are those equivalents? tried to google their comparison, but no relevant
> results found for PostgreSQL

I don't know; it somewhat depends on how smart the planner is which is out
of my league.  I would expect that NOT EXISTS is typically a better first
option since EXCEPT needs to do sorting and de-duplicating (maybe?) of large
amounts of data while the NOT EXISTS method seems to require some level of
nested looping to process but only needs to find a single matching record to
return false so less memory constraints.

Someone more familiar with the internals may be able to give a more detailed
answer from the top of their head.

But in a critical (or under-performing) piece of code you should probably
test both to see in your reality which one perform better as I would guess
hardware is going to have an impact.

David J.


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