Structured Text Retrieval Models

Structured text retrieval models provide a formal definition or mathematical framework for querying semistructured textual databases. A textual database contains both content and structure. The content is the text itself, and the structure divides the database into separate textual parts and relates those textual parts by some criterion. Often, textual databases can be represented as marked up text, for instance as XML, where the XML elements define the structure on the text content. Retrieval models for textual databases should comprise three parts: 1) a model of the text, 2) a model of the structure, and 3) a query language [4]: The model of the text defines a tokenization into words or other semantic units, as well as stop words, stemming, synonyms, etc. The model of the structure defines parts of the text, typically a contiguous portion of the text called element, region, or segment, which is defined on top of the text modelâ\u80\u99s word tokens. The query language typically defines a number of operators on content and structure such as set operators and operators like â\u80\u9ccontaining â\u80\u9d and â\u80\u9ccontained-by â\u80\u9d to model relations between content and structure, as well as relations between the structural elements themselves. Using such a query language, the (expert) user can for instance formulate requests like â\u80\u9cI want a paragraph discussing formal models near to a table discussing the differences between databases and information retrievalâ\u80\u9d. Here, â\u80\u9cformal models â\u80\u9d and â\u80\u9cdifferences between databases and information retrieval â\u80\u9d should match the content that needs to be retrieved from the database, whereas â\u80\u9cparagraph â\u80\u9d and â\u80\u9ctable â\u80\u9d refer to structural constraints on the units to retrieve. The features, structuring power, and the expressiveness of the query languages of several models for structured text retrieval are discussed below. HISTORICAL BACKGROUND The STAIRS system (Storage and Information Retrieval System), which was developed at IBM already in the late 1950â\u80\u99s allowed querying both content and structure. Much like todayâ\u80\u99s On-line Public Access Catalogues, it wa