Re: Minor mathematical error in documentation

From: Peter Eisentraut <peter_e(at)gmx(dot)net>
To: pgsql-bugs(at)postgresql(dot)org
Cc: Russell Smith <mr-russ(at)pws(dot)com(dot)au>
Subject: Re: Minor mathematical error in documentation
Date: 2008-01-17 19:56:02
Message-ID: 200801172056.02664.peter_e@gmx.net
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Russell Smith wrote:
> SELECT ... WHERE CASE WHEN x <> 0 THEN y/x > 1.5 ELSE false END;
>
> A CASE construct used in this fashion will defeat optimization attempts,
> so it should only be done when necessary. (In this particular example,
> it would be best to sidestep the problem by writing y > 1.5*x instead.)
>
>
> In-equality transformations do not guarantee that y > 1.5x == y/x >
> 1.5. This is only true for x>0

So the proper expression would be

SELECT ... WHERE CASE WHEN x >= 0 THEN y > 1.5*x ELSE y < 1.5*x END;

or

SELECT ... WHERE (x >= 0 AND y > 1.5*x) OR y < 1.5*x;

which obviously isn't simpler. So I suggest that we just delete the
parenthetical note.

--
Peter Eisentraut
http://developer.postgresql.org/~petere/

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