1. 2005rel-xml-ii1 The SilkRoute system  The system goals  Scenario, examples  View Forests  View forest and query composition  View forest efficient execution

2. 2005rel-xml-ii2  The system goals Publishing of relational data in xml form Efficient execution of XQuery queries on published views Assumptions: The only real data is the relational data The public XML view is virtual XQuery queries on published views are composed with view def. Execution is always by SQL queries, since relational engines are efficient

3. 2005rel-xml-ii3  Scenario, examples The scenario: e-commerce Each supplier has a private relational store This store can be represented in a canonical XML view The format/contents of both is private There is a public XML schema def, that all participants in the scenario know about and use – the public view Resellers, merchants pose queries on the public view, as per their current interests

4. 2005rel-xml-ii4 Supplier private data: rel and XML (fig. 2, p. 5, fig.4, p. 8)

5. 2005rel-xml-ii5 The generation of the canonical view from the relational data is standard, can be completely automated But, the db is not converted to the XML view – doing this will throw away the availability of SQL for queries The canonical view is virtual And, it is just a basis for further work; this data is considered private For the public, there is an XML schema agreed upon by the community – the public view

6. 2005rel-xml-ii6 The public view schema: (fig. 1, p. 5) back10 back14back10back14 XQuery type syntax – an internal format (more human readable) of XML, used by the XQuery type checker Differences: one root, mainly products, change of names of some fields, reports inside product

7. 2005rel-xml-ii7 The db admin can now write queries on the canonical view (p. 8, p. 9) $CanonicalView is a pre-defined variable, bound to the canonical view

8. 2005rel-xml-ii8 A more complex query, with fusion (fig. 5, p. 10)

9. 2005rel-xml-ii9 Note: XQuery is compositional: Given Q1 on a a source S, Q2 on results of Q1, there exists a query Q3 on S, such that Q3(S) = Q2(Q1(S)) Simple approach: define Q1 in a let, then use it as in Q2 (see prev. page for let) Here (next page) is a query defining our supplier’s public view (ignore boxes) (fig. 6, p. 11)

10. 2005rel-xml-ii10 public view

11. 2005rel-xml-ii11 Users (e.g., resellers) may pose queries on the public view e.g. somebody wants only the deeply discounted items (fig.7) This can be composed with the def. of the public view An efficient composition, that does not generate intermediate results, and retrieves only what is needed, is more difficult

12. 2005rel-xml-ii12 Here is an efficient composition (as might be produced by an optimizer) (fig. 8, p. 13)

13. 2005rel-xml-ii13  View Forests But, the canonical view is also virtual – all data is in a relational db How do we represent XML virtual views over relational, so that composition can be performed? The representation in SilkRoute: View forest (typically a tree, but XQuery allows XML sequences, hence forest) A tree structure – the structure of the XML view Nodes are labeled XML labels, and with SQL fragments – these are used to generate the data

14. 2005rel-xml-ii14 public view back17

15. 2005rel-xml-ii15 Explanations: Each node is assigned a (Dewey style) id, no explicit tree is maintained The XML label of an internal node is an element/attribute The XML label of a leaf is an atomic type (string, float, …) Each node is labeled with an SQL fragment, with FROM, WHERE, SELECT components (may be empty, not always shown) –A leaf must have a SELECT –Every variable used in a node, must be defined (in FROM) in the node or in an ancestor

16. 2005rel-xml-ii16 The SQL query for a node: C(n) The union of the fragments for the nodes on the path from the node to the root Examples: N1.1.1: N1.2: SELECT * (select an empty result tuple) FROM clothing c WHERE c.category = “outerwear” N1.2.1.1: N1.2.5.1: Note: if n is parent of n’, then C(n’) is an extension of C(n) (possibly the same, possibly a new var, or a new condition…)

17. 2005rel-xml-ii17 The semantics of a view forest: Given a (relational) instance I, view forest V, V(I) is: Collection of nodes (n, r) where –n is a node of V –r is a row in the answer of C(n) on I (n, r) is parent of (n’,r’) if –n is parent of n’ in V –r, r’ are obtained from variables assignments that agree on all the variables of C(n) Look on the view (p. 14) and nodesp. 14 N1.1.1, N.1.2.1.1, N1.2.5.1

18. 2005rel-xml-ii18 The semantics  a naïve, inefficient method for generating the XML represented by a view V from instance I More efficient method: combine many SQL queries into one Will be discussed later (2 nd half of paper)

19. 2005rel-xml-ii19 Here is a view forest for a fragment of the canonical view Can be generated automatically from db schema