XML Querying and Views

1. XML Querying and Views Helena Galhardas DEI IST (slides baseados na disciplina CIS 550 – Database & Information Systems, Univ. Pennsylvania, Zachary Ives)

2. Agenda • Recalling XML Querying • Views

3. XQuery’s Basic Form Has an analogous form to SQL’s SELECT..FROM..WHERE..GROUPBY..ORDER BY The model: bind nodes (or node sets) to variables; operate over each legal combination of bindings; produce a set of nodes “FLWOR” statement [note case sensitivity!]: for {iterators that bind variables} let {collections} where {conditions} order by {order-conditions} (older version was “SORTBY”) return {output constructor} 3

4. “Iterations” in XQuery A series of (possibly nested) FOR statements assigning the results of XPaths to variables for $root in document(“http://my.org/my.xml”) for $sub in $root/rootElement, $sub2 in $sub/subElement, … Something like a template that pattern-matches, produces a “binding tuple” For each of these, we evaluate the WHERE and possibly output the RETURN template document() or doc() function specifies an input file as a URI Old version was “document”; now “doc” but it depends on your XQuery implementation 4

5. Example XML Data attribute root p-i element Root text dblp ?xml mastersthesis article mdate mdate key key author title year school 2002… editor title journal volume year ee ee 2002… 1992 1997 The… ms/Brown92 tr/dec/… PRPL… Digital… db/labs/dec Univ…. Paul R. Kurt P…. SRC… http://www. 5

6. Two XQuery Examples <root-tag> { for $p in document(“dblp.xml”)/dblp/proceedings, $yr in $p/yr where $yr = “1999” return <proc> {$p} </proc> } </root-tag> for $i in document(“dblp.xml”)/dblp/inproceedings[author/text() = “John Smith”] return <smith-paper> <title>{ $i/title/text() }</title> <key>{ $i/@key }</key> { $i/crossref } </smith-paper> 6

7. Another Example attribute root p-i element Root text universities ?xml university … mastersthesis country name key … author title year school Univ…. USA 1999 ms/Brown92 Univ…. PRPL… Kurt P…. 7

8. What If Order Doesn’t Matter? By default: SQL is unordered XQuery is ordered everywhere! But unordered queries are much faster to answer XQuery has a way of telling the query engine to avoid preserving order: unordered { for $x in (mypath) …} 8

9. Querying & Defining Metadata – Can’t Do This in SQL Can get a node’s name by querying node-name(): for $x in document(“dblp.xml”)/dblp/* return node-name($x) Can construct elements and attributes using computed names: for $x in document(“dblp.xml”)/dblp/*, $year in $x/year, $title in $x/title/text(), element node-name($x) { attribute {“year-” + $year} { $title } } 9

10. XQuery Wrap-up XQuery is very SQL-like, but in some ways cleaner and more orthogonal It is based on paths and binding tuples, with collections and trees as its first-class objects See www.w3.org/TR/xquery/ for more details on the language 10

11. A Problem We frequently want to reference data in a way that differs from the way it’s stored XML data  HTML, text, etc. Relational data  XML data Relational data  Different relational representation XML data  Different XML representation Generally, these can all be thought of as different viewsover the data A view is a named query Let’s start with a special presentation language for XML  HTML 11

12. XSL(T): XML  “Other Stuff” XSL (XML Stylesheet Language) is actually divided into two parts: XSL:FO: formatting for XML XSLT: a special transformation language We’ll leave XSL:FO for you to read off www.w3.org, if you’re interested XSLT is actually able to convert from XML  HTML, which is how many people do their formatting today Products like Apache Cocoon generally translate XML  HTML on the server side Your browser will do XML  HTML on the client side 12

13. Other Forms of Views XSLT is a language primarily designed from going from XML  non-XML Obviously, we can do XML  XML in XQuery … Or relations  relations … What about relations  XML and XML  relations? Let’s start with XML  XML, relations  relations 13

14. Views in SQL and XQuery A view is a named query We use the name of the view to invoke the query (treating it as if it were the relation it returns) SQL: CREATE VIEW V(A,B,C) AS SELECT A,B,C FROM R WHERE R.A = “123” XQuery:declare function V() as element(content)* { for $r in doc(“R”)/root/tree, $a in $r/a, $b in $r/b, $c in $r/c where $a = “123” return <content>{$a, $b, $c}</content> } Using the views: SELECT * FROM V, RWHERE V.B = 5 AND V.C = R.C for $v in V()/content, $r in doc(“r”)/root/treewhere $v/b = $r/breturn $v 14

15. What’s Useful about Views Providing security/access control We can assign users permissions on different views Can select or project so we only reveal what we want! Can be used as relations in other queries Allows the user to query things that make more sense Describe transformations from one schema (the base relations) to another (the output of the view) The basis of converting from XML to relations or vice versa This will be incredibly useful in data integration, discussed soon… Allow us to define recursive queries 15

16. Materialized vs. Virtual Views A virtual view is a named query that is actually re-computed every time – it is merged with the referencing query CREATE VIEW V(A,B,C) AS SELECT A,B,C FROM R WHERE R.A = “123” A materialized view is one that is computed once and its results are stored as a table Think of this as a cached answer These are incredibly useful! Techniques exist for using materialized views to answer other queries Materialized views are the basis of relating tables in different schemas SELECT * FROM V, RWHERE V.B = 5 AND V.C = R.C 16

17. Views Should Stay Fresh Views (sometimes called intensional relations) behave, from the perspective of a query language, exactly like base relations (extensional relations) But there’s an association that should be maintained: If tuples change in the base relation, they should change in the view (whether it’s materialized or not) If tuples change in the view, that should reflect in the base relation(s) 17

18. View Maintenance and the View Update Problem There exist algorithms to incrementally recompute a materialized view when the base relations change We can try to propagate view changes to the base relations However, there are lots of views that aren’t easily updatable: We can ensure views are updatable by enforcing certain constraints (e.g., no aggregation),but this limits the kinds of views we can have delete? R R⋈S S 18

19. Views as a Bridge between Data Models A claim made several times: “XML can’t represent anything that can’t be expressed in in the relational model” If this is true, then we must be able to represent XML in relations Store a relational view of XML (or create an XML view of relations) 19

20. An Important Set of Questions Views are incredibly powerful formalisms for describing how data relates: fn: rel …  rel rel (Or XML XML XML, or rel rel  XML, ...) Can I define a view recursively? • Why might this be useful in the XML construction case? When should the recursion stop? Suppose we have two views, v1 and v2 • How do I know whether they represent the same data? • If v1 is materialized, can we use it to compute v2? • This is fundamental to query optimization and data integration, as we’ll see later

21. Reasoning about Queries and Views SQL or XQuery are a bit too complex to reason about directly • Some aspects of it make reasoning about SQL queries undecidable We need an elegant way of describing views (let’s assume a relational model for now) • Should be declarative • Should be less complex than SQL • Doesn’t need to support all of SQL – aggregation, for instance, may be more than we need

22. Referências • Raghu Ramakrishnan et al, “Database Management Systems” • Avi Silberchatz et al, “Database System Concepts” • XML Recommendation.