Re: RM and abstract syntax trees

David Cressey wrote:

> "David BL" <davidbl_at_iinet.net.au> wrote in message
> news:1194653707.402224.177270_at_s15g2000prm.googlegroups.com...
>

>>On Nov 10, 7:29 am, "David Cressey" <cresse..._at_verizon.net> wrote:
>>
>>
>>>One of the fundamental properties of a pointer is that you can

>>>it by using it as an address in an address space,  in order to retrieve

>>>thing pointed to directly.   There is no corresponding property in
>>>sequentially generated arbitrary meaningless identifiers.
>>
>>The mathematician in me axiomatizes the concept of pointer in terms of
>>its abstract properties, which (ignoring null pointers) are
>>
>>    1) the existence of an associated address space, which is just a
>>set of objects
>>    2) a bijection between objects in the address space and pointer
>>values
>>        In one direction this bijection is "address of" and in the
>>other
>>        it is "dereference"
>>    3) the ability to compare pointer values
>>
>>This encompass physical address spaces, virtual address spaces, C++
>>smart pointers, persistent OIDs and pointer swizzling etc.
>>
>>It also encompass node identifiers of an AST if we regard the DB as
>>defining an address space of AST nodes, and an appropriate select
>>query represents a dereference that logically binds to precisely one
>>node of the AST.
>>
>>My perspective conflicts with your statement above.

I would argue that an OID is nothing more than an abstract address. The problem with OIDs are people use them to point at things. The first and most compelling indication of this is when they mention some alleged performance advantage.

> In most pointer based systems, the pointer is not included in the contents,
> being seen as redundant to its location.
>
> The distinction between content based addressing and pointer based
> addressing is fundamental to the comparison of databases built on the
> relational model and databases built on the graph model.

The products that used OIDs generally used them as pointers.

> It is possible, by suitable abstraction and by poor design choices, to
> create a database that offers none of the advantages claimed for the
> relational model, while appearing to conform to the relational model
> superficially. Sad to say, thousands of such databases have been
> constructed over the years, and have led many uneducated database newbies
> to believe that the relational model doesn't really offer the advantages
> that its proponents cliam.

Hear! Hear! Received on Sat Nov 10 2007 - 22:10:51 CET