Chapter 19: Information Retrieval Database System Concepts ©Silberschatz, Korth and Sudarshan See www. db-book. com for conditions on re-use 1
Chapter 19: Information Retrieval n Relevance Ranking Using Terms n Relevance Using Hyperlinks n Synonyms. , Homonyms, and Ontologies n Indexing of Documents n Measuring Retrieval Effectiveness n Web Search Engines n Information Retrieval and Structured Data n Directories Database System Concepts - 5 th Edition, Sep 2, 2005 19. 2 ©Silberschatz, Korth and Sudarshan
Information Retrieval Systems n Information retrieval (IR) systems use a simpler data model than database systems l Information organized as a collection of documents l Documents are unstructured, no schema n Information retrieval locates relevant documents, on the basis of user input such as keywords or example documents l e. g. , find documents containing the words “database systems” n Can be used even on textual descriptions provided with non-textual data such as images n Web search engines are the most familiar example of IR systems Database System Concepts - 5 th Edition, Sep 2, 2005 19. 3 ©Silberschatz, Korth and Sudarshan
Information Retrieval Systems (Cont. ) n Differences from database systems l IR systems don’t deal with transactional updates (including concurrency control and recovery) l Database systems deal with structured data, with schemas that define the data organization l IR systems deal with some querying issues not generally addressed by database systems 4 Approximate 4 Ranking searching by keywords of retrieved answers by estimated degree of relevance Database System Concepts - 5 th Edition, Sep 2, 2005 19. 4 ©Silberschatz, Korth and Sudarshan
Keyword Search n In full text retrieval, all the words in each document are considered to be keywords. We use the word term to refer to the words in a document n Information-retrieval systems typically allow query expressions formed using keywords and the logical connectives and, or, and not l Ands are implicit, even if not explicitly specified l n Ranking of documents on the basis of estimated relevance to a query is critical l Relevance ranking is based on factors such as 4 Term frequency – Frequency of occurrence of query keyword in document 4 Inverse document frequency – How many documents the query keyword occurs in » Fewer give more importance to keyword 4 Hyperlinks to documents – More links to a document is more important Database System Concepts - 5 th Edition, Sep 2, 2005 19. 5 ©Silberschatz, Korth and Sudarshan
Relevance Ranking Using Terms n TF-IDF (Term frequency/Inverse Document frequency) ranking: l Let n(d) = number of terms in the document d l n(d, t) = number of occurrences of term t in the document d. l Relevance of a document d to a term t TF (d, t) = log 4 The l n(d, t) 1+ n(d) log factor is to avoid excessive weight to frequent terms Relevance of document to query Q r (d, Q) = TF (d, t) t Q n(t) Database System Concepts - 5 th Edition, Sep 2, 2005 19. 6 ©Silberschatz, Korth and Sudarshan
Relevance Ranking Using Terms (Cont. ) n Most systems add to the above model l Words that occur in title, author list, section headings, etc. are given greater importance l Words whose first occurrence is late in the document are given lower importance l Very common words such as “a”, “an”, “the”, “it” etc are eliminated 4 Called l stop words Proximity: if keywords in query occur close together in the document, the document has higher importance than if they occur far apart n Documents are returned in decreasing order of relevance score l Usually only top few documents are returned, not all Database System Concepts - 5 th Edition, Sep 2, 2005 19. 7 ©Silberschatz, Korth and Sudarshan
Similarity Based Retrieval n Similarity based retrieval - retrieve documents similar to a given document l Similarity may be defined on the basis of common words 4 E. g. find k terms in A with highest TF (d, t ) / n (t ) and use these terms to find relevance of other documents. n Relevance feedback: Similarity can be used to refine answer set to keyword query l User selects a few relevant documents from those retrieved by keyword query, and system finds other documents similar to these n Vector space model: define an n-dimensional space, where n is the number of words in the document set. Vector for document d goes from origin to a point whose i th coordinate is TF (d, t ) / n (t ) l The cosine of the angle between the vectors of two documents is used as a measure of their similarity. l Database System Concepts - 5 th Edition, Sep 2, 2005 19. 8 ©Silberschatz, Korth and Sudarshan
Relevance Using Hyperlinks n Number of documents relevant to a query can be enormous if only term frequencies are taken into account n Using term frequencies makes “spamming” easy 4 E. g. a travel agency can add many occurrences of the words “travel” to its page to make its rank very high n Most of the time people are looking for pages from popular sites n Idea: use popularity of Web site (e. g. how many people visit it) to rank site pages that match given keywords n Problem: hard to find actual popularity of site l Solution: next slide Database System Concepts - 5 th Edition, Sep 2, 2005 19. 9 ©Silberschatz, Korth and Sudarshan
Relevance Using Hyperlinks (Cont. ) n Solution: use number of hyperlinks to a site as a measure of the popularity or prestige of the site l Count only one hyperlink from each site (why? - see previous slide) l Popularity measure is for site, not for individual page 4 But, most hyperlinks are to root of site 4 Also, concept of “site” difficult to define since a URL prefix like cs. yale. edu contains many unrelated pages of varying popularity n Refinements l When computing prestige based on links to a site, give more weight to links from sites that themselves have higher prestige 4 Definition 4 Set l is circular up and solve system of simultaneous linear equations Above idea is basis of the Google Page. Rank ranking mechanism Database System Concepts - 5 th Edition, Sep 2, 2005 19. 10 ©Silberschatz, Korth and Sudarshan
Relevance Using Hyperlinks (Cont. ) n Connections to social networking theories that ranked prestige of people l E. g. the president of the U. S. A has a high prestige since many people know him l Someone known by multiple prestigious people has high prestige n Hub and authority based ranking l A hub is a page that stores links to many pages (on a topic) l An authority is a page that contains actual information on a topic l Each page gets a hub prestige based on prestige of authorities that it points to l Each page gets an authority prestige based on prestige of hubs that point to it l Again, prestige definitions are cyclic, and can be got by solving linear equations l Use authority prestige when ranking answers to a query Database System Concepts - 5 th Edition, Sep 2, 2005 19. 11 ©Silberschatz, Korth and Sudarshan
Synonyms and Homonyms n Synonyms l E. g. document: “motorcycle repair”, query: “motorcycle maintenance” 4 need l to realize that “maintenance” and “repair” are synonyms System can extend query as “motorcycle and (repair or maintenance)” n Homonyms l E. g. “object” has different meanings as noun/verb l Can disambiguate meanings (to some extent) from the context n Extending queries automatically using synonyms can be problematic l Need to understand intended meaning in order to infer synonyms 4 Or l verify synonyms with user Synonyms may have other meanings as well Database System Concepts - 5 th Edition, Sep 2, 2005 19. 12 ©Silberschatz, Korth and Sudarshan
Concept-Based Querying n Approach l For each word, determine the concept it represents from context l Use one or more ontologies: 4 Hierarchical 4 E. g. : structure showing relationship between concepts the ISA relationship that we saw in the E-R model n This approach can be used to standardize terminology in a specific field n Ontologies can link multiple languages n Foundation of the Semantic Web (not covered here) Database System Concepts - 5 th Edition, Sep 2, 2005 19. 13 ©Silberschatz, Korth and Sudarshan
Indexing of Documents n An inverted index maps each keyword Ki to a set of documents Si that contain the keyword l Documents identified by identifiers n Inverted index may record l Keyword locations within document to allow proximity based ranking l Counts of number of occurrences of keyword to compute TF n and operation: Finds documents that contain all of K 1, K 2, . . . , Kn. l Intersection S 1 S 2 . . . Sn n or operation: documents that contain at least one of K 1, K 2, …, Kn l union, S 1 S 2 . . . Sn, . n Each Si is kept sorted to allow efficient intersection/union by merging l “not” can also be efficiently implemented by merging of sorted lists Database System Concepts - 5 th Edition, Sep 2, 2005 19. 14 ©Silberschatz, Korth and Sudarshan
Measuring Retrieval Effectiveness n Information-retrieval systems save space by using index structures that support only approximate retrieval. May result in: l false negative (false drop) - some relevant documents may not be retrieved. l false positive - some irrelevant documents may be retrieved. l For many applications a good index should not permit any false drops, but may permit a few false positives. n Relevant performance metrics: l precision - what percentage of the retrieved documents are relevant to the query. l recall - what percentage of the documents relevant to the query were retrieved. Database System Concepts - 5 th Edition, Sep 2, 2005 19. 15 ©Silberschatz, Korth and Sudarshan
Measuring Retrieval Effectiveness (Cont. ) n Recall vs. precision tradeoff: 4 Can increase recall by retrieving many documents (down to a low level of relevance ranking), but many irrelevant documents would be fetched, reducing precision n Measures of retrieval effectiveness: l Recall as a function of number of documents fetched, or l Precision as a function of recall 4 Equivalently, l as a function of number of documents fetched E. g. “precision of 75% at recall of 50%, and 60% at a recall of 75%” n Problem: which documents are actually relevant, and which are not Database System Concepts - 5 th Edition, Sep 2, 2005 19. 16 ©Silberschatz, Korth and Sudarshan
Web Search Engines n Web crawlers are programs that locate and gather information on the Web l Recursively follow hyperlinks present in known documents, to find other documents 4 Starting l from a seed set of documents Fetched documents 4 Handed 4 Can over to an indexing system be discarded after indexing, or store as a cached copy n Crawling the entire Web would take a very large amount of time l Search engines typically cover only a part of the Web, not all of it l Take months to perform a single crawl Database System Concepts - 5 th Edition, Sep 2, 2005 19. 17 ©Silberschatz, Korth and Sudarshan
Web Crawling (Cont. ) n Crawling is done by multiple processes on multiple machines, running in parallel l Set of links to be crawled stored in a database l New links found in crawled pages added to this set, to be crawled later n Indexing process also runs on multiple machines l Creates a new copy of index instead of modifying old index l Old index is used to answer queries l After a crawl is “completed” new index becomes “old” index n Multiple machines used to answer queries l Indices may be kept in memory l Queries may be routed to different machines for load balancing Database System Concepts - 5 th Edition, Sep 2, 2005 19. 18 ©Silberschatz, Korth and Sudarshan
Information Retrieval and Structured Data n Information retrieval systems originally treated documents as a collection of words n Information extraction systems infer structure from documents, e. g. : l Extraction of house attributes (size, address, number of bedrooms, etc. ) from a text advertisement l Extraction of topic and people named from a new article n Relations or XML structures used to store extracted data l System seeks connections among data to answer queries l Question answering systems Database System Concepts - 5 th Edition, Sep 2, 2005 19. 19 ©Silberschatz, Korth and Sudarshan
Directories n Storing related documents together in a library facilitates browsing l users can see not only requested document but also related ones. n Browsing is facilitated by classification system that organizes logically related documents together. n Organization is hierarchical: classification hierarchy Database System Concepts - 5 th Edition, Sep 2, 2005 19. 20 ©Silberschatz, Korth and Sudarshan
A Classification Hierarchy For A Library System Database System Concepts - 5 th Edition, Sep 2, 2005 19. 21 ©Silberschatz, Korth and Sudarshan
Classification DAG n Documents can reside in multiple places in a hierarchy in an information retrieval system, since physical location is not important. n Classification hierarchy is thus Directed Acyclic Graph (DAG) Database System Concepts - 5 th Edition, Sep 2, 2005 19. 22 ©Silberschatz, Korth and Sudarshan
A Classification DAG For A Library Information Retrieval System Database System Concepts - 5 th Edition, Sep 2, 2005 19. 23 ©Silberschatz, Korth and Sudarshan
Web Directories n A Web directory is just a classification directory on Web pages l E. g. Yahoo! Directory, Open Directory project l Issues: 4 What should the directory hierarchy be? 4 Given a document, which nodes of the directory are categories relevant to the document l Often done manually 4 Classification of documents into a hierarchy may be done based on term similarity Database System Concepts - 5 th Edition, Sep 2, 2005 19. 24 ©Silberschatz, Korth and Sudarshan
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