1. Views: Alternate Data Representations Zachary G. Ives University of Pennsylvania CIS 550 – Database & Information Systems November 2, 2004 Some slide content courtesy of Susan Davidson, Dan Suciu, & Raghu Ramakrishnan

2. 2 Administrivia  One- to two-page project plan document due Thursday  Today: your first paper for reviewing  See the handout on writing a paper review  Please post the (short) review to the newsgroup, before class next Tuesday

3. 3 Where We Left Off…  Wrapping up a discussion of XQuery  Very flexible and powerful language for XML  Clean and orthogonal: can always replace a collection with an expression that creates collections  DB and document-oriented (we hope)  The core is relatively clean and easy to understand  One feature worth noting, which had no correspondence in the SQL world…

4. 4 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 } }

5. 5 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 views over the data  A view is a named query  Let’s start with a special presentation language for XML  HTML

6. 6 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 interestedwww.w3.org  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

7. 7 A Different Style of Language  XSLT is based on a series of templates that match different parts of an XML document  There’s a policy for what rule or template is applied if more than one matches (it’s not what you’d think!)  XSLT templates can invoke other templates  XSLT templates can be nonterminating (beware!)  XSLT templates are based on XPath “match”es, and we can also apply other templates (potentially to “select”ed XPaths)  Within each template, we describe what should be output  (Matches to text default to outputting it)

8. 8 An XSLT Stylesheet This is DBLP …

9. 9 Results of XSLT Stylesheet Paper1 Smith Chakrabarti Gray Paper2 This Is DBLP Paper1 Smith Paper2 Chakrabarti Gray

10. 10 What XSLT Can and Can’t Do  XSLT is great at converting XML to other formats  XML  diagrams in SVG; HTML; LaTeX  …  XSLT doesn’t do joins (well), it only works on one XML file at a time, and it’s limited in certain respects  It’s not a query language, really  … But it’s a very good formatting language  Most web browsers (post Netscape 4.7x) support XSLT and XSL formatting objects  But most real implementations use XSLT with something like Apache Cocoon – compatible with more browsers  You should use XSL/XSLT to format the forms and pages for your projects – see www.w3.org/TR/xslt or the chapter we handed out for more detailswww.w3.org/TR/xslt

11. 11 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

12. 12 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 $a = “123” return {$a, $b, $c} } SELECT * FROM V, R WHERE V.B = 5 AND V.C = R.C for $v in V()/content, $r in doc(“r”)/root/tree where $v/b = $r/b return $v Using the views:

13. 13 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

14. 14 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, R WHERE V.B = 5 AND V.C = R.C

15. 15 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)

16. 16 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 AB 12 22 BC 24 23 R S ABC 124 123 224 223 R⋈SR⋈S delete?

17. 17 Views as a Bridge between Data Models A claim we’ve 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)

18. 18 A View as a Translation between XML and Relations  You’ll be reviewing the most-cited paper in this area (Shanmugasundaram et al), and there are many more (Fernandez et al., …)  Techniques already making it into commercial systems  XPERANTO at IBM Research will appear in DB2 v8 or 9  SQL Server has some XML support; Oracle is also doing some XML  … Now you’ll know how it works!

19. 19 Issues in Mapping Relational  XML  We know the following:  XML is a tree  XML is SEMI-structured  There’s some structured “stuff”  There is some unstructured “stuff”  Issues relate to describing XML structure, particularly parent/child in a relational encoding  Relations are flat  Tuples can be “connected” via foreign-key/primary-key links

20. 20 The Simplest Way to Encode a Tree  You saw this one in your midterm: XYZ 14  If we have no IDs, we CREATE values…  BinaryLikeEdge(key, label, type, value, parent) keylabeltypevalueparent 0treeref-- 1contentref-0 2sub- content ref-1 3i-contentref-1 4-strXYZ2 5-int143 What are shortcomings here?

21. 21 Florescu/Kossmann Improved Edge Approach  Consider order, typing; separate the values  Edge(parent, ordinal, label, flag, target)  Vint(vid, value)  Vstring(vid, value) parentordlabelflagtarget -1treeref0 01contentref1 11sub-contentstrv2 11i-contentintv3 vidvalue v314 vidvalue v2XYZ

22. 22 How Do You Compute the XML?  Assume we know the structure of the XML tree (we’ll see how to avoid this later)  We can compute an “XML-like” SQL relation using “outer unions” – we first this technique in XPERANTO  Idea: if we take two non-union-compatible expressions, pad each with NULLs, we can UNION them together  Let’s see how this works…

23. 23 A Relation that Mirrors the XML Hierarchy  Output relation would look like: rLabelridrOrdclabelcidcOrdsLabelsidsOrdstrint tree01-------- -01content11----- -01-11sub-content21-- -01-11-21XYZ- -01-12i-content31-- -01-12-31-14

24. 24 A Relation that Mirrors the XML Hierarchy  Output relation would look like: rLabelridrOrdclabelcidcOrdsLabelsidsOrdstrint tree01-------- -01content11----- -01-11sub-content21-- -01-11-21XYZ- -01-12i-content31-- -01-12-31-14

25. 25 A Relation that Mirrors the XML Hierarchy  Output relation would look like: rLabelridrOrdclabelcidcOrdsLabelsidsOrdstrint tree01-------- -01content11----- -01-11sub-content21-- -01-11-21XYZ- -01-12i-content31-- -01-12-31-14 Colors are representative of separate SQL queries…

26. 26 SQL for Outputting XML  For each sub-portion we preserve the keys (target, ord) of the ancestors  Root: select E.label AS rLabel, E.target AS rid, E.ord AS rOrd, null AS cLabel, null AS cid, null AS cOrd, null AS subOrd, null AS sid, null AS str, null AS int from Edge E where parent IS NULL  First-level children: select null AS rLabel, E.target AS rid, E.ord AS rOrd, E1.label AS cLabel, E1.target AS cid, E1.ord AS cOrd, null AS … from Edge E, Edge E1 where E.parent IS NULL AND E.target = E1.parent

27. 27 The Rest of the Queries  Grandchild: select null as rLabel, E.target AS rid, E.ord AS rOrd, null AS cLabel, E1.target AS cid, E1.ord AS cOrd, E2.label as sLabel, E2.target as sid, E2.ord AS sOrd, null as … from Edge E, Edge E1, Edge E2 where E.parent IS NULL AND E.target = E1.parent AND E1.target = E2.parent  Strings: select null as rLabel, E.target AS rid, E.ord AS rOrd, null AS cLabel, E1.target AS cid, E1.ord AS cOrd, null as sLabel, E2.target as sid, E2.ord AS sOrd, Vi.val AS str, null as int from Edge E, Edge E1, Edge E2, Vint Vi where E.parent IS NULL AND E.target = E1.parent AND E1.target = E2.parent AND Vi.vid = E2.target  How would we do integers?

28. 28 Finally…  Union them all together: ( select E.label as rLabel, E.target AS rid, E.ord AS rOrd, … from Edge E where parent IS NULL) UNION ( select null as rLabel, E.target AS rid, E.ord AS rOrd, E1.label AS cLabel, E1.target AS cid, E1.ord AS cOrd, null as … from Edge E, Edge E1 where E.parent IS NULL AND E.target = E1.parent ) UNION (. : ) UNION (. : )  Then another module will add the XML tags, and we’re done!

29. 29 “Inlining” Techniques  Folks at Wisconsin noted we can exploit the “structured” aspects of semi-structured XML  If we’re given a DTD, often the DTD has a lot of required (and often singleton) child elements  Book(title, author*, publisher)  Recall how normalization worked:  Decompose until we have everything in a relation determined by the keys  … But don’t decompose any further than that  Shanmugasundaram et al. try not to decompose XML beyond the point of singleton children

30. 30 Inlining Techniques  Start with DTD, build a graph representing structure tree content sub-content i-content * * * The edges are annotated with ?, * indicating repetition, optionality of children They simplify the DTD to figure this out @id ?

31. 31 Building Schemas  Now, they tried several alternatives that differ in how they handle elements w/multiple ancestors  Can create a separate relation for each path  Can create a single relation for each element  Can try to inline these  For tree examples, these are basically the same  Combine non-set-valued things with parent  Add separate relation for set-valued child elements  Create new keys as needed name book author

32. 32 Schemas for Our Example  TheRoot(rootID)  Content(parentID, id, @id)  Sub-content(parentID, varchar)  I-content(parentID, int)  If we suddenly changed DTD to <!ELEMENT content(sub-content*, i-content?) what would happen?

33. 33 XQuery to SQL  Inlining method needs external knowledge about the schema  Needs to supply the tags and info not stored in the tables  We can actually directly translate simple XQuery into SQL over the relations – not simply reconstruct the XML

34. 34 An Example for $X in document(“mydoc”)/tree/content where $X/sub-content = “XYZ” return $X  The steps of the path expression are generally joins  … Except that some steps are eliminated by the fact we’ve inlined subelements  Let’s try it over the schema: TheRoot(rootID) Content(parentID, id, @id) Sub-content(parentID, varchar) I-content(parentID, int)

35. 35 Wrapping up…  We’ve seen that views are useful things  Allow us to store and refer to the results of a query  We’ve seen an example of a view that changes from XML to relations – and we’ve even seen how such a view can be posed in XQuery and “unfolded” into SQL  Next time: we’ll start reasoning about views, which involves the introduction of another query language!