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List: sqlite-users
Subject: Re: [sqlite] SQLite vs. Oracle (parallelized)
From: Chris Wedgwood <cw () f00f ! org>
Date: 2009-02-26 17:24:06
Message-ID: 20090226172406.GA685769 () puku ! stupidest ! org
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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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