# Re: insert to projection

From: Vadim Tropashko <vadimtro_at_gmail.com>

Date: Sun, 6 Sep 2009 11:28:29 -0700 (PDT)

Message-ID: <680406b5-cab1-4930-8083-05e728f381a5_at_12g2000pri.googlegroups.com>

On Sep 4, 9:58 am, paul c <toledobythe..._at_oohay.ac> wrote: > ... We can delete from projection ...

dy = [p]

Received on Sun Sep 06 2009 - 13:28:29 CDT

Date: Sun, 6 Sep 2009 11:28:29 -0700 (PDT)

Message-ID: <680406b5-cab1-4930-8083-05e728f381a5_at_12g2000pri.googlegroups.com>

On Sep 4, 9:58 am, paul c <toledobythe..._at_oohay.ac> wrote: > ... We can delete from projection ...

Are you suggesting projection being updatable in respect to deletion? I fail to see this, here is my worksheet:

Let x be the base relation,

z an empty relation with subset of attributes of x,
y = x v z the projection of x onto attributes of z,
dx decrement of x,

dy decrement of y.

Counterexample:

dx = [p q]

1 a 2 a

*;*dy = [p]

2

*;*

z = [p]

*;*

x = [p q]

1 a 1 b 2 a

*;*QBQL assertion that generated it (against Figure1.db database):

z != [] & -- that is z != DUM. Introduced after QBQL -- found counterexample with z = [] z ^ [] = z & -- z is an empty relation (aka header) --x v z = y & -- View def: y is projection of x onto z -- Substituted y with (x v z) everywhere -- to make the counterexample search faster x ^ [] = dx ^ [] & -- x and dx have the same header (x v z) ^ [] = dy ^ [] & -- y and dy have the same header z > x ^ [] & -- Atributes of z are contained in attributes of x dx < x & -- decrement relation containment dy < x v z & -- ditto (x ^ dx') v z = (x v z) ^ dy' -- View definition after update -> dx = dy ^ x. -- decrement of the base in terms of -- decrements of the view and baserelation

If you are uncomfortable with deletion of more than one tuple at once, then here is modified single tuple counterexample:

dx = [p q]

1 a

*;*

dy = [p]

*;*

z = [p]

*;*

x = [p q]

1 a 1 b 2 a

*;*Received on Sun Sep 06 2009 - 13:28:29 CDT