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From: Tom Lane on 22 Jun 2010 00:02
Robert Haas <robertmhaas(a)gmail.com> writes:
> ...but I'm still having a hard time wrapping my head around it.

The fundamental point is to be able to force a value to go to NULL
when outer-join logic says it ought to. Consider

CREATE VIEW foo AS SELECT x,y,'zed' FROM bar;

SELECT * FROM baz LEFT JOIN foo ON (baz.a = foo.x);

If you try to flatten the view then you end up with a constant 'zed'
that needs to be replaced by NULL whenever baz.a hasn't got a match
in bar.x. There's no way to make a constant go to NULL though: it's
a constant, n'est-ce pas? Instead, we have the idea of an expression
PlaceHolderVar(foo, 'zed'). This will go to null if variables from foo
ought to go to null. Otherwise it produces 'zed'. Sort of an
anti-COALESCE.

regards, tom lane

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