From sqlite-users  Thu Feb 26 17:24:06 2009
From: Chris Wedgwood <cw () f00f ! org>
Date: Thu, 26 Feb 2009 17:24:06 +0000
To: sqlite-users
Subject: Re: [sqlite] SQLite vs. Oracle (parallelized)
Message-Id: <20090226172406.GA685769 () puku ! stupidest ! org>
X-MARC-Message: https://marc.info/?l=sqlite-users&m=123573796220345

On Thu, Feb 26, 2009 at 09:53:18AM -0600, Jay A. Kreibich wrote:

>   I've always wondered about this... someone please correct me if I'm
>   wrong, but my understanding is that there wasn't any difference
>   between a left and right join except for the argument order.  It
>   seems like implementing right joins should be pretty easy by just
>   having the parser reverse the arguments when it builds the parse
>   tree.  I realize that might screw with the optimizer a bit, but is
>   there anything obvious I'm missing?

When you use a right and a left join together you can't do that.
Sometimes you can restructure the queries to make use or two left
joins or a sub-query, but it's often cumbersome and not always the same
thing.

If in general there is a way to turn something like

  select c1,c2,c3 from m left join l on l.mid=m.id right join r on l.rid=r.id

into something conceptually the same that only uses left joins then
perhaps the optimizer could do this for us.

There might be corner cases though were such a transform isn't
possible.  When it's vague like this I defer to people like Igor who's
SQL abilities constantly amaze me.
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