Re: delete from jon
Date: Mon, 07 Sep 2009 02:55:53 GMT
Message-ID: <ZC_om.1393$Jd7.963_at_nwrddc02.gnilink.net>
"paul c" <toledobythesea_at_oohay.ac> wrote in message
news:_zAom.42880$Db2.42221_at_edtnps83...
> Walter Mitty wrote:
> ....
>> Part of the problem with regard to sloppy language is that the term "view
>> update" is misleading. If view C is defined as A join B and one were to
>> apply an update, let's say MINUS D, what gets updated? If we were, in
>> reality, updating view C, then the update would be really simple. We
>> would update view C by changing its definition. The new definition of C
>> is (A join B) minus D. This might require making D be persistent, so
>> that the view can be applied later. Hey, presto! View C has just been
>> updated!
>>
>> But that's not what we really mean when we say "update view C". What we
>> mean is "leave view C defined exactly as before, but update A and B such
>> that the effect on C's apparent extension is the same as if the update
>> had been applied to a base relvar whose extension is the same as C's
>> apparent extension." Under this meaning of "update view C" the operation
>> is underconstrained, as has already been noted.
>
> I changed the subject back to delete from join because the above is not
> about insert to projection. They are very different problems (unlike
> delete from join, insert to projection is not just a matter of
> constraints).