Re: Lucid statement of the MV vs RM position?

From: Jon Heggland <jon.heggland_at_idi.ntnu.no>
Date: Sat, 06 May 2006 21:48:53 +0200
Message-ID: <e3iuj3$mqg$1@orkan.itea.ntnu.no>

Bob Badour wrote:

> Jon Heggland wrote:

>> Maybe I was unclear. TTM has such an aggregate operator; it is called
>> UNION. You could define GROUP as SUMMARIZE ... ADD (UNION(...) AS
>> RVA)---provided you have this mechanism for "converting" a set of
>> attributes to a singleton relation---but GROUP is still not an aggregate
>> operator. UNION is. In other words, UNION is to (eh...) UNION as SUM is
>> to +.

>
> Actually, GROUP is to UNION as SUM is to +

Not the Tutorial D GROUP that I'm talking about. What's the point of just restating Marshall's claim without addressing anything I said?

Perhaps I still was unclear; let me try again. Obviously you can postulate an aggregate operator that defined as iterated union, like SUM is iterated addition. Tutorial D does just that, and calls it (perhaps confusingly) UNION. You could call it GROUP instead, but Tutorial D does not. It uses the name GROUP for a unary relation operator that is shorthand for a particular extension/projection; alternatively a summarisation using that iterated union aggregate operator. I honestly don't see why this is so difficult to grasp.

>>> I don't see this as any different than performing a type conversion
>>> before performing the addition for SUM.
>>
>> I think we are just quibbling over Tutorial D syntax, or over which
>> operators should be defined in terms of which others.
>> R GROUP ({X} AS Y) is a valid expression;
>> R SUM (X AS Y) is not.
>> SUMMARIZE R BY { ALL BUT X } ADD (SUM(X) AS Y) is a valid expression;
>> SUMMARIZE R BY { ALL BUT X } ADD (GROUP(X) AS Y) is not.

> 
> Is your complaint that Tutorial D is not orthogonal? Or that it uses a
> different syntax for GROUP ?

I have no complaint. These example were meant to show how the usage of the GROUP operator is different from that of SUM. GROUP is an aggregate operator no more than EXTEND is.

-- 
Jon

Received on Sat May 06 2006 - 14:48:53 CDT