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values of a given set of paths. On the contrary to the relational counterpart, an XFD can have a non-text
valued path on its right-hand side, and XFD can be considered in an XML subtree denoted by a context
path. XFDs can be specified by a class of tree-pattern formulas.
Query Reformulation: Query reformulation is a process in which a query ( Q ) formulated against
a target schema ( T ) is rewritten to a form conforming to a source schema ( S ), using a mapping ( m )
from S to T . Query reformulation implements the virtual data integration as opposed to data exchange
in materialized data integration. However, in both cases answer to Q must be the same, i.e. Q ( m ( I S )) =
ref m ( Q )( I S ), where m ( I S ) transforms an instance of schema S into an instance of schema T , and ref m ( Q )
reformulates query Q into a query over schema S .
Query Propagation: Query propagation is a process of sending a query across a network of peer-to-
peer connected nodes along semantic paths determined by schema mappings between schemas of peer
databases. A peer receives a query from one of its partners (acquaintances), reformulates it and sends
forward (propagates) to all other partners. The answer obtained from the peer's database is then merged
with answers obtained from partners to whom the query was propagated. The result is next sent back to
the partner who delivered the query.
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