Databases Reference
In-Depth Information
when new variable constraints are inserted or a user inserts such constraints. When
identifiers and equality constraints are used, their effective domains become
important, however we assume it global for simplicity.
As in the definition of a graph, an identifier is defined for one label. Let
id
and
lab
be functions to extract an identifier and a label from a given graph, for graphs
and
lab
(
91
)
≠
lab
(
92
)
are inconsistent. That is, the definitions
of become dummy (
⊥
). In other cases, each attribute should be merged as follows:
•
In a case of
l-attr
,
a same label must have the same value, otherwise
inconsistent.
l-attr
's with different labes are merged.
•
In a case of
,
a constructor should be same. In a case of set, merging
is required, while, in a case of list, concatenation is required.
e-attr
For example, when
g
1
and
g
2
is merged into
Its result,
e-attr,
is also merged if it satisfies the above conditions, we can consider
to enhance the possibility of merging by introducing type concepts as in
-term
Ait-
Kaci 84)
In a case of equality constraints, when
the following merge is executed
for a given constraint . This operation is required when objects which had
be considered to be different are identified, or an equation is given by constraint
propagation. We can get maximum information by taking minimum graphs.
4.5
Query Transformation for Document Groups
A document group guarantees that a query returns the same result, even if they
consist of different documents. For the purpose, it is necessary to transform a
query to a set of queries and a summation operation, depending on physical
configuration
Kunishima et al 99
Consider a query
σ
F
(g)
(selection) for a document group
in Section 3.2.5. It is transformed into
,
where
is a summation operation.
