tailieunhanh - Keyword Search in Databases- P5

Keyword Search in Databases- P5:Conceptually, a database can be viewed as a data graph GD(V ,E), where V represents a set of objects, and E represents a set of connections between objects. In this book, we concentrate on two kinds of databases, a relational database (RDB) and an XML database. In an RDB, an object is a tuple that consists of many attribute values where some attribute values are strings or full-text; there is a connection between two objects if there exists at least one reference from one to the other | 14 2. SCHEMA-BASED KEYWORD SEARCH ON RELATIONAL DATABASES Figure Rightmost path expansion The algorithm allows adding an arbitrary edge to an arbitrary position in a partial tree when expanding line 9-13 which makes the number of temporal results extremely large while only few of them will contribute to the final results. This is because most of the results will end up with a partial tree that is of size Tmax but does not contain all keywords total . For example for Tmax 3 and Q Michelle XML over the database with schema graph shown in Figure many will stop expansion in line 6 of Algorithm 1 such as T A Michelle N W N P . The algorithm needs a large number of tree isomorphism tests which is costly. This is because the isomorphism test will only be performed when a valid MTJNT is generated. As a result all isomorphisms of an MTJNT will be generated and checked. For example MTJNT A Michelle N W N P XML can be generated through various ways such as A Michelle A Michelle N W A Michelle N W N P XML and P XML W N P XML A Michelle N W N P XML . In order to solve the above problems S-KWS Markowetz et al. 2007 proposes an algorithm 1 to reduce the number of partial results generated by expanding from part of the nodes in a partial tree and 2 to avoid isomorphism testing by assigning a proper expansion order. The solutions are based on the following properties Property-1 For any partial tree we can always find an expansion order where every time a new edge is added into the rightmost root-to-leaf path of the tree. An example for the rightmost expansion is shown in Figure where a tree of size 7 is expanded by adding an edge to the rightmost path of the tree each time. Property-2 Every leaf node must contain a unique keyword if it is not on the rightmost root-to-leaf path of a partial tree. This is based on the rightmost path expansion discussed above. A leaf node which is not on the rightmost path of a partial tree will not be further expanded in other words it .