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Re: Nested Sets vs. Nested Intervals

From: vc <boston103_at_hotmail.com>
Date: 10 Nov 2005 12:30:57 -0800
Message-ID: <1131654657.221904.311330@g47g2000cwa.googlegroups.com>

amado.alves_at_netcabo.pt wrote:
> I'll not give an algebra for RDF here since RAL is readily available
> and I have my own algebra to worry about. So I'll give you a sketch of
> the latter instead.
>
> Mneson is a graph-based metamodel of data, based on a graph with the
> following properties:
>
> 1. Connections are binary, directed, and untyped i.e. unlabelled. Note
> that being untyped entails not having an explicit identifier. A
> connection is identified solely by the (ordered) pair of vertices it
> represents. Hence, there are no duplicate connections. That is, for any
> pair of vertices (x, y) there can be at most one connection from x to
> y. For a connection from x to y we say that x is a "source" of y and y
> is a "target" of x.
>
> 2. A vertex is either a datum or a pivot.
>
> (a) A data vertex or datum represents an atomic value of data e.g. an
> integer, a real number, a symbol. The value of a data vertex is the
> identity of the vertex.
>
> (b) A pivot has no intrinsic value but it has nevertheless a unique
> identity. In practice a serial number is used to identify pivot
> vertices.
>
> 3. There are no island vertices. There may be, however, island
> subgraphs.
>
> A database instance is expressed in the base structure by means of
> special subgraph topologies that represent complex data entities like
> attributes, lists, sets, etc. [1] A possible definition is as follows:
>
> Attribute. An attribute of name x1 and value x2 is represented by an
> attribute instance x3 such that x1 connects to x3 and x3 connects to
> x2. The attribute is connected to its subject x0 by connecting x0 to
> x3. For example suppose that x5 represents an employee; the fact that
> x5 has Social Security Number, or SSN (the attribute name) 123456789
> (the attribute value) is expressed by connecting: x5 to x6, "SSN" to
> x6, x6 to 123456789.

Do you mean to say that the entity is presented as a graph whose nodes are values of some type and edges are directed paths with two paths (S and T) per each node ? If so, how is it different or better that the traditional network model ?

>
> Set. For a vertex x that represents a set, the targets of x represent
> the members of the set. In parlance we say x is the 'head' of the set.
> Note this is a different object than the algebraic type set of vertices
> below.
>
> Etc.
>
> The topologies may overlap, e.g. a vertex x that is an attribute type
> may also be the member of a set, or even a set itself in which case the
> respective attribute instance is a member of x.
>
> An algebra (called the Mneson Calculus) is defined on the base graph as
> follows. The algebraic type is the set of vertices. The operations
> include the set operations (intersection, union, etc.) and the (unary)
> adjacency operators T, S for Targets, Sources, with the obvious
> semantics:
>
> T (X) = {y : y is a target of x, x in X}
> S (X) = {y : y is a source of x, x in X}
>
> The Mneson Calculus serves as a query language. For example, the
> following query retrieves the employee with SSN 123456789:
>
> T ({Employees}) ^ S [T ({"SSN"}) ^ S({123456789})]
>
> where ^ means set intersection.
>
> Singleton arguments are very common so we take the liberty of ommiting
> the {}, for clarity; and we want to focus on algebraic issues so often
> we also ommit the "" around literals. With these simplifications the
> expression above becomes
>
> T (Employees) ^ S [T (SSN) ^ S (123456789)]
>
> This example of course assumes a schema where vertex Employees is the
> head of the set of employees.
> ________
> [1] These entities are also called "semantic" structures because they
> are designed to model semantic domains. Note how the semantic
> structures are in turn modelled in terms of the base untyped graph. So
> formally we have a chain of domains here and we say "semantic domain"
> do describe the former level. Sometimes I say "semantic domain" even
> when it is not necessary to distinguish, for habit I guess, I am sorry
> if I sound pedantic, please just ignore the word on those occasions.
Received on Thu Nov 10 2005 - 14:30:57 CST

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