Re: Concurrency in an RDB

paul c wrote:

> Bob Badour wrote:
> ...
>

>> What I am saying is: When you project onto A, the data type of B is 
>> mostly** irrelevant. Likewise, when you project onto B, the data type 
>> of A is mostly irrelevant.
>>
>> The fact that you have a recursive data type definition has no effect 
>> on project or join or restrict or union or intersect or difference 
>> etc. The values identified as B are simply values.
>>
>> Assuming:
>>
>> A = { a1, a2, a3, a4, a5 }
>> B = { {a,b} | a in A and b in B }
>>
>> Given relation R{a in A,b in B}: /* Using C-style comments */
>>
>> R = { { a1, { a2, { a3, {} } } }  /* a=a1, b={ a2, { a3, {} } */
>>   , { a4, { a3, {} } }            /* a=a4, b={ a3, {} } */
>>   , { a5, { a2, { a3, {} } } }    /* a=a5, b={ a2, { a3, {} } */
>> }
>>
>>
>> Project R onto R1=R{A}:
>>
>> R1 = { { a1 }, { a4 }, { a5 } }
>>
>> Project R onto R2=R{B}:
>>
>> R2 = { { a2, { a3, {} } }, { a3, {} } }
>>
>>
>> ** I say "mostly" irrelevant because the predicate for the projected 
>> relation still makes reference to B in the sense of "A where exists 
>> some B".

It states something different. Whether it is useful depends on the predicate of the relation. I had not noticed when I created the example that I associate a3 with {} in each tuple. Nothing says that is necessary. I could just as easily created the example as:

R = { { a1, { a2, { a3, {} } } } /* a=a1, b={ a2, { a3, {} } */

   , { a4, { a3, {} } } /* a=a4, b={ a3, {} } */    , { a5, { a3, { a2, {} } } } /* a=a5, b={ a3, { a2, {} } */ }

R1 = R{a} = { { a1 }, { a4 }, { a5 } }
R2 = R{b} = { { a2, { a3, {} } }, { a3, {} }, { a3, { a2 {} } } }

   Or is it redundant?

It is not redundant.

   One way I look at
> this is to imagine that the value a3 stands in the same relation to {}
> as a2 stands to { a3, {} } or put another way a3 has the same relation
> to {} as a2 has to { a3, {} }. But in the tuple
>
> { { a1, { a2, { a3, {} } } } /* a=a1, b={ a2, { a3, {} } */
>
> is it not already implicit that a3 stands in that same relation to {}?

As I clarified above, nothing says a3 always stands in the same relation to {}. That was merely incidental in my example.

> Eg., why bother with the fourth tuple? If I go without the inserted
> (fourth) tuple, then by itself, it must be false, ie., it must be true
> in the logical complement of R. So the first tuple implies that a3 has
> relation R to {} but the complement or R says it doesn't, which looks
> like a contradiction to me!

Why bother? Only bother if it is or becomes true.

> Maybe this is nonsense, but I'd to know why!

I think my original example misled you. Received on Sat Dec 23 2006 - 15:47:08 CET