Re: unnesesary sorting after Merge Full Join - Mailing list pgsql-general

On Tue, 2008-02-26 at 10:34 -0500, Tom Lane wrote:
> > and add COALESCE(id2,id1) to it as well I think.
>
> No, because those two expressions are not equivalent.  (Hmm ... squint
> ... but full merge join is pretty much symmetric, so it's not clear
> why it should matter which side is left or right.  Maybe COALESCE
> isn't
> exactly the right concept with which to describe the merged variable?)

It is, in this case only, since when id2 is not null id2 == id1.

So in this case its OK to express a symmetric relationship as a left
handed function.

--
  Simon Riggs
  2ndQuadrant  http://www.2ndQuadrant.com