Re: Rules and Views

Curt Sampson <cjs@cynic.net> writes:
> On Thu, 1 Aug 2002, Tom Lane wrote:
>> Curt Sampson <cjs@cynic.net> writes:
> You want to be careful with this sort of stuff, since the query planner
> sometimes won't do the view as efficiently as it would do the fully
> specified equivalant query. I've posted about this here before.
>> 
>> Please provide an example.  AFAIK a view is a query macro, and nothing
>> else.

> I already did provide an example, and you even replied to it. :-)

But that isn't an "equivalent query".  You've manually transformed   SELECT * FROM (SELECT something UNION SELECT
somethingelse)WHERE foo;
 
into   (SELECT something WHERE foo) UNION (SELECT somethingelse WHERE foo);
As has been pointed out repeatedly, it's not entirely obvious whether
this is a valid transformation in the general case.  (The knee-jerk
reaction that it's obviously right should be held in check, since SQL's
three-valued notion of boolean logic tends to trip up the intuition.)
If you can provide a proof that it's always safe, or that it's safe
under such-and-such conditions, I'll see what I can do about making it
happen.
        regards, tom lane