# Re: Idempotence and "Replication Insensitivity" are equivalent ?

Date: 21 Sep 2006 14:22:51 -0700

Message-ID: <1158873771.593808.115480_at_k70g2000cwa.googlegroups.com>

Chris Smith wrote:

> If you have an n-ary relation of the form A1 x A2 x A3 x ... x An, then

*> there are 2^n - 1 possible projections (excluding the project that
**> selects no columns, because it's useless; but quite arbitrarily
**> including the identity projection, which is just the original relation).
*

Not useless!

For any relation, the projection over 0 attributes results in a relation that is either TABLE_DEE if the relation has members, TABLE_DUM otherwise. Since DEE and DUM are easily interpreted as boolean values, we can think of the project over zero attributes as a function "isNotEmpty".

But perhaps I shouldn't be piping up in any discussion where 2^n-1 is involved. Ahem.

Marshall Received on Thu Sep 21 2006 - 23:22:51 CEST