6 research outputs found
Optimization of query plans in the presence of access limitations
We consider the problem of querying data sources that have
limited capabilities and can thus only be accessed by complying with
certain binding patterns for their attributes. This is often the case, e.g.,
in the context of data on the web queryable via web forms as well as
in legacy data wrapped in relational tables. In such contexts, computing
the answer to a user query cannot be done as in a traditional database;
instead, a query plan is needed that takes the access limitations into
account.
In this paper, we develop a technique for producing a (possibly recursive) Datalog program that retrieves all obtainable answers for a query
with limited source capabilities. In particular, we improve with respect
to a previously published algorithm for optimizing query answering for
conjunctive queries. Furthermore, we extend it to the context of unions
of conjunctive queries. The algorithm exploits the structure of the query
together with the binding patterns present in the source schema in order
to compute an optimized query plan. The optimization excludes from the
query plan the sources that are not relevant for the answer thus reducing
the number of accesses to the sources