CMPSCI Homework 4

Transcription

1 CMPSCI Homework 4 Due May 1 st, before class Exercise 1. XPath Containment (10 points) Containment and equivalence are important decision problems for XML query languages, just as they are for relational query languages. Suppose we have two XPath expressions X1 and X2. The evaluation of X1 on XML document D is X1(D), a set of nodes from D (unordered). We define containment as follows: X1 X2 if for all XML documents D, X1(D) X2(D). If X1 X2 and X2 X1, then X1 and X2 are equivalent expressions. Reason informally about whether containment holds below: X1: /alpha[*]/beta X2: /alpha//beta X3: /alpha[beta][gamma] X4: /alpha[gamma][beta] X5: /alpha[beta]/*//gamma X6: /alpha[beta]//*/gamma a) does X1 X2 hold? b) does X2 X1 hold? c) does X3 X4 hold? d) does X4 X3 hold? e) does X5 X6 hold? f) does X6 X5 hold? Exercise 2. Minimal XPath expressions (10 points) Restricting our attention to the simple XPath features used above (child axis, descendent-or-self axis, simple predicates, and wildcard *) an XPath expression is minimal if all equivalent XPath expressions are at least as big (counting all node tests, including *, and predicates). a) Are any of the queries X1.. X6 above non-minimal? If so, give a smaller equivalent expression. b) Is there a single unique minimal form for any XPath query? Explain why or give a counterexample. c) Minimize the following the XPath expression: /a[b/*][b//g]/b/g

2 Exercise 3. Querying XML data sets (70 points) In this exercise, you will write queries over two XML data sets using the XQuery language. You will execute your queries using the open-source Saxon system, which we have installed in the class directory space on the EdLab machines: /courses/cs600/cs645/cs645/hw/hw4/src/saxonb8-7-1j In order to run Saxon, take the following steps: 1) Basic setup Go to your home directory, make a subdirectory hw4, and set your $PATH and $CLASSPATH variables. > mkdir hw4 > cd hw4 > export PATH=/exp/rcf/edlab/jdk1.5.0_01/bin/:$PATH > export CLASSPATH=/courses/cs600/cs645/cs645/hw/hw4/src/saxonb8-7-1j/: /courses/cs600/cs645/cs645/hw/hw4/src/saxonb8-7-1j/saxon8.jar:$classpath 2) Data and query files The two data sets bib.xml and dblp.xml are placed under the following path: /courses/cs600/cs645/cs645/hw/hw4/data/ For each question below, compose an XQuery query. Use the doc() function in your query to address the data file. For example, the following query retrieves all book elements from the bib.xml file. doc( /courses/cs600/cs645/cs645/hw/hw4/data/bib.xml )/bib/book Store each query in a separate file. Follow the naming convention of Qi.xq for the i th query, e.g., Q1.xq for the first query, Q2.xq for the second, and so on. 3) Query execution After a query is composed, run it using the following command: > java net.sf.saxon.query Q1.xq In this assignment, you are required to store each query result in a separate file. To do so, type: > java net.sf.saxon.query Q1.xq > Q1.res Submission After you finish this exercise, you should have the following files generated: (Qi.xq, Qi.res), i = 1, 2, 3, 4, 5, 6, 7. Create a tar file using the following command: > tar -cvf {YOUR_NAME}-hw4.tar Q?.* Please the tar file to cs645-help@edlab-mail.cs.umass.edu before class on the May 1 st. Also, when you hand in your solutions in class, include the queries. We will look at them to give partial credit if your query results are not completely correct. Note 1: You can setup Saxon on your own machine and try some queries there. But, please generate the final query results on the EdLab machines, as UNIX and Windows have different format for text files. Note 2: We will be grading your solutions primarily on correctness. However, we may mark down for very contorted queries, as in XQuery such queries often imply inefficient solutions. As a guideline, most of your queries should run in a few seconds up to a few minutes. If the execution time of your query exceeds 10 minutes, we recommend you looking for a simpler expression of the query.

3 Queries over bib.xml The first XML data set you will be running queries over is a bibliography file. The DTD of this file, bib.dtd, is available in the same directory as the data. The DTD can be tremendously helpful to you in understanding the structure of the data and formulating your queries. A visualized presentation of the dtd is also provided in the data directory under the path livedtd-bib/index.html. You can navigate through the hyperlinks to familiarize yourself with the elements and attributes in this DTD. 1. List books that were published after 1991 and cost less than $100. <book > <book > 2. List books that were published after 1991 and cost less than $100, including their year, title, and price. The output format should be: <cheap-book year=""> <title></title> <price></price> </cheap-book> <cheap-book year=""> <title></title> <price></price> </cheap-book> 3. Find books in which the name of some author contains the string "Stev". For each such book, return the title and the qualifying author. <book > <title></title> <author></author> <book > <title></title> <author></author> 4. For each publisher, list its name and all books that it has published; each book listed should contain the following child elements in the specified order: year, title, and authors. You can use string() or data() to convert an attribute value to the text content of a new element, e.g., <new-element> string($b/@attribute) </new-element>. <publisher> <name></name>

4 <book> <year></year> <title></title> <author></author> <author></author> <book>. </publisher> <publisher> </publisher>. 5. Create a flat list of all the (title, author) pairs; the title and author in each pair should belong to the same book. Enclose each pair in a "title-author" element. Hint: you can define multiple variables in the for clause, e.g., for $x in, $y in, $z in where return. <title-author> <title></title> <author><author> </title-author> <title-author> <title></title> <author><author> </title-author> Queries over dblp.xml The second XML data set is a DBLP file containing all the publications of ACM SIGMOD up to year The DTD of this file, dblp.dtd, is available in the same directory as the data. The dtd can also be visualized using livedtd-dblp/index.html. This DTD is more complex than the previous one. You may want to browse the DTD and the data file at the same time to get familiar with the elements and attributes used in the data. Note that the proceedings elements define the proceedings and the inproceedings elements refer to the papers included in the proceedings. Each inproceedings element contains a key attribute that uniquely identifies the paper across all SIGMOD publications. An inproceedings element can contain multiple cite elements, each referring to another inproceedings element by including in its text content the key value of the referenced element. 6. Retrieve those inproceedings elements that are referenced more than 20 times and return them in descending order of the reference count. For each returned element, create a new element paper to contain the title, author(s), year, and reference count.

5 The output format should be: <most-cited> <paper> <title></title> <author></author> <author></author> <year></year> <reference-count></reference-count> </paper> </most-cited> 7. Return those authors that have more than 20 papers (i.e., inproceedings elements) in descending order of the paper count. For each qualifying author, create a new element researcher to contain his/her name, the paper count, and a list of papers in ascending order of the year of publication; each paper in the list has a year attribute and its text content is the paper title. The output format should be: <paper-machines> <researcher> <name></name> <paper-count></paper-count> <paper year= >title goes here</paper> <paper year= >title goes here</paper> </researcher> </paper-machines>

6 Exercise 4. Data security and inference (10 points) Suppose the table Employee(Name, Age, Department, Salary) is protected, but two views computed from the Employee table are available. V1 selects Name and Age for all employees. V2 groups by Department and computes the minimum and maximum for both Age and Salary attributes. You are given the following output for V1 and V2: Name Age Alice 20 Bob 25 Carol 30 Dave 30 Edgar 40 Dept Min(age) Max(age) Min(salary) Max(salary) Accounting Marketing a) Assuming each employee works in only one department, derive lower and upper bounds for each employee s salary. b) Now suppose employees may work in more than one department. Do any of your answers to part (a) change? c) Are there instances of the Employee table, with the same employee Names, in which each employee s salary is exactly disclosed by the publication of V1 and V2?