Onderwerpen

1
Onderwerpen

• Typering van Graaf-manipulatie operatoren
• Jan Hidders Typing Graph-Manipulation
  Operations. ICDT 2003 394-409 2002
• Lockingmechanismen voor XML
• Stijn Dekeyser, Jan Hidders A Commit Scheduler
  for XML Databases. APWeb 2003 83-88
• Stijn Dekeyser, Jan Hidders Path Locks for XML
  Document Collaboration. WISE 2002 105-114
• Xpath/Xquery query optimalisatie
• Jan Hidders, Philippe Michiels Avoiding
  Unnecessary Ordering Operations in XPath. DBPL
  2003
• Xpath satisfiability
• Jan Hidders Satisfiability of XPath expressions.
  DBPL 2003

2
Query Optimization 1/3

• Explicit sorting and duplicate elimination after
  each step by inserting distinct-docorder (ddo)
  operations in query plan

• Semantics of implementation without ddos
  sloppy semantics

• Example /descendant-or-selfa/childb

3
Query Optimization 2/3
4
Query Optimization 3/3

• Our automaton does not consider ddo-operations in
  de query plan.
• e.g. child/foll-sibl/child
• if after every step a sorting operation is
  performed, there is no need for sorting at the
  end of the expression.
• How to decide where to sort?
• e.g. /descendant-or-selfa/childb/parenta
• if ddo after child step then ddo at end required,
  otherwise the result is sorted and only a linear
  dup-elim is required.

5
Satisfiability 1/3

• Satisfiability
• Result of XPath expression not always empty?
• Related to
• Avoiding duplicate elimination and ordering
• Subsumption problem
• Examples
• selfa/selfb
• childa/child/parentb
• /child/parent/parent
• /preceding
• /following

6
Satisfiability 2/3

• Data Model
• Alphabet of tag names S
• XML tree ordered node-labeled tree T
• Document order preorder tree walk

XPath Expressions (subset XPath 2.0) P
A S A P/P / PP P ? P P ? P P
P. A self parent child anc-or-self
desc-or-self foll-sibl prec-sibl. Note
desc ? desc-or-self/child foll ?
anc-or-self/foll-sibl/desc-or-self Fragments
P/,, ? ,P?, ?,
7
Satisfiability 3/3

• Sat. of TDGs is NP-complete.
• Xpathfragments

• Open problems
• upperbound P-
• complexity P? and P/,?
• relationship TDGs and P/,,?