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Re: [HACKERS] SQL compatibility reminder: MySQL vs PostgreSQL

From: "Paragon Corporation" <lr(at)pcorp(dot)us>
To: "'Josh Berkus'" <josh(at)agliodbs(dot)com>,"'Joshua Waihi'" <josh(at)catalyst(dot)net(dot)nz>
Cc: "'PostgreSQL Advocacy'" <pgsql-advocacy(at)postgresql(dot)org>
Subject: Re: [HACKERS] SQL compatibility reminder: MySQL vs PostgreSQL
Date: 2010-03-08 04:16:50
Message-ID: 674F31537E484E08869F9EAD82FD065D@J (view raw, whole thread or download thread mbox)
Lists: pgsql-advocacypgsql-hackers
  I've heard this complaint from MySQL users switching to PostgreSQL and SQL
Server  before. Even heard it from FoxPro users which also seems to have
this non-standard behavior of group by.

A simple DISTINCT ON doesn't quite do it for them.  I think you need to wrap
it in a subselect to get the equivalent meaning.

If I am not mistaken in MySQL when you have a construct such as

SELECT u.username, n.subject,n.note, n.post_date
FROM users As u INNER JOIN notes As n ON u.user_id = n.user_id
GROUP BY u.username
ORDER BY n.post_date DESC

It essentially means return the username, subject and note of the last note
for each user and sort by the last post date

The problem they say with DISTINCT ON  is that it forces sorting by the key
first, which is not what they want.

So the equivalent I think would be

(SELECT DISTINCT ON(u.username) u.username, n.subject, n.note, n.post_date
FROM users As  u INNER JOIN notes As n ON u.user_id = n.user_id
ORDER BY u.username, n.post_date DESC) As foo
ORDER BY foo.post_date DESC

Hope that helps,

-----Original Message-----
From: pgsql-advocacy-owner(at)postgresql(dot)org
[mailto:pgsql-advocacy-owner(at)postgresql(dot)org] On Behalf Of Josh Berkus
Sent: Sunday, March 07, 2010 6:05 PM
To: Joshua Waihi
Cc: Mark Kirkwood; francois(dot)perou(at)free(dot)fr; Craig Ringer; Andrew Dunstan;
PostgreSQL Advocacy; dpage(at)pgadmin(dot)org
Subject: Re: [pgsql-advocacy] [HACKERS] SQL compatibility reminder: MySQL vs


I've moved this discussion to pgsql-advocacy where it belongs.

> Aside from Drupal core, its too soon to know if there will be problems 
> with D7 contrib but there are a few major bugs about other D6 contrib 
> modules. The biggest one, which I think need movement to get fixed is 
> in the Views module. The Views module has a bit of a hard time trying 
> to please both databases and its surrounding the use of DISTINCT and 
> restricting duplicate results. They've opted for a solution that 
> really hits hard on PostgreSQL's performance. Bascially, when a 
> DISTINCT clause is used, all other fields being selected get a custom 
> functional called FIRST rapped around them: SELECT DISTINCT(nid), 
> FIRST(title), FIRST(body), ..... The function merely returns the first 
> value when two values are present for that row. This is the alternate 
> instead of grouping by each field. Its stupid and needs to be fixed. 
> The issue is
> here:

What's the obstacle to fixing it?  It seems like just changing to SELECT
DISTINCT ON ... would do the trick, and would allow cutting code.

--Josh Berkus

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