XML & Databases. Tutorial. 9. Staircase Join, Symmetries. Universität Konstanz. Database & Information Systems Group Prof. Marc H.

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1 XML & Databases Tutorial Christian Grün, Database & Information Systems Group University of, Winter 2007/08

2 Pre/Post Encoding <html> <head> <title>xml</title> </head> <body> <h1>databases & XML</h1> <div> <b>assignments</b> <ul> <li>exercise 1</li> <li>exercise 2</li> </ul> </div> </body> </html> Seite 2

3 Pre/Post Encoding The Pre/Post plane forms regions which are equivalent to the four major axes: ancestor following preceding descendant Example current context node <b> Seite 3

4 Pre/Post Encoding As shown in the lecture, the XPath axes can be rewritten: v' v/ancestor: pre(v') < pre(v) pos(v') > pos(v) v' v/following: pre(v') > pre(v) pos(v') > pos(v) v' v/preceding: pre(v') < pre(v) pos(v') < pos(v) v' v/descendant: pre(v') > pre(v) pos(v') < pos(v) Seite 4

5 Sequential Traversal Suppose we have three context nodes and try to find their descendant-or-self nodes: (6, 11, 13)/descendant-or-self::node() we sequentially parse the table nodes and check the post values Staircase Join nodes might be parsed multiple times or can t be part of the result set algorithm can optimized by Pruning, Partitioning and Skipping Seite 5

6 Staircase Join Pruning observation: some nodes in the initial node set are descendants of other nodes pruning means: all nodes are ignored which appear in the region of other context nodes before the traversal starts, the pruning algorithm removes these nodes out of the initial context set in our example, node 13 can be safely removed Seite 6

Staircase Join Partitioning observation: some nodes will unnecessarily be scanned multiple times we can limit our scanning process to the unique plane regions as our context sets

7 Staircase Join Partitioning observation: some nodes will unnecessarily be scanned multiple times we can limit our scanning process to the unique plane regions as our context sets will always be ordered, we can stop as soon as the next context node is encountered in our example, we only scan the nodes with the pre values 6 to 10 (11-1) and 11 to 15 Seite 7

Staircase Join Skipping observation: as soon as a post value is bigger than the initial value, all following post values will be bigger (see Pre/Post plane) we

8 Staircase Join Skipping observation: as soon as a post value is bigger than the initial value, all following post values will be bigger (see Pre/Post plane) we can stop parsing as soon as the current post value exceeds the initial value in our example, we can limit our traversal to the nodes 6 to 8 and 11 to 15 Seite 8

9 Staircase Join Pruning On-The-Fly Pruning was treated as a pre-processing step if we sequentially parse the Pre/Post data, we can include prune nodes on the fly if the next context node equals the currently parsed node, it is simply ignored in our example, node 13 is skipped during the traversal Seite 9

10 Tree Structure 1 html 15 Post Values 2 head 3 5 body title h1 5 8 div 13 <html> <head> <title>xml</title> </head> <body> <h1>databases & XML</h1> <div> <b>assignments</b> <ul> <li>exercise 1</li> <li>exercise 2</li> </ul> </div> </body> </html> 4 XML Databases & XML 9 10 b 7 6 Assignments li ul li Exercise 1 Exercise 2 Seite 10

Pre/Parent Encoding The Pre/Parent plane looks pretty different, but allows similar operations: Pruning: ignore nodes with a larger parent value Partitioning: only depends on Pre value

11 Pre/Parent Encoding The Pre/Parent plane looks pretty different, but allows similar operations: Pruning: ignore nodes with a larger parent value Partitioning: only depends on Pre value same as Pre/Post Skipping: stop traversal as soon as parent gets smaller than parent of context node in our example, we parse the same nodes as for Pre/Post: 6 to 8 and 11 to 15 Seite 11

12 Tree Structure 1 0 html Parent Values 2 head body title h1 8 5 div <html> <head> <title>xml</title> </head> <body> <h1>databases & XML</h1> <div> <b>assignments</b> <ul> <li>exercise 1</li> <li>exercise 2</li> </ul> </div> </body> </html> 4 3 XML 7 6 Databases & XML 9 10 b 9 8 Assignments 12 li ul li Exercise 1 Exercise 2 Seite 12

Part XVII. Staircase Join Tree-Aware Relational (X)Query Processing. Torsten Grust (WSI) Database-Supported XML Processors Winter 2008/09 440

Part XVII. Staircase Join Tree-Aware Relational (X)Query Processing. Torsten Grust (WSI) Database-Supported XML Processors Winter 2008/09 440 Part XVII Staircase Join Tree-Aware Relational (X)Query Processing Torsten Grust (WSI) Database-Supported XML Processors Winter 2008/09 440 Outline of this part 1 XPath Accelerator Tree aware relational