url - Universität zu Lübeck

5.4. QUERY PROCESSING 95
1 Index getIndex ( IndexCollection indexes , Xpath q , StatDataGuide sdg ) {
2 subsetIndexes = new List ( ) ;
4 Kq = q . getKeyPaths ;
5 Qq = q . getQualifierPaths ;
6 vq = q . getReturnValuePath ;
8 f o r a l l index j indexes{
9 indexdecl = index . getDeclaration ( ) ;
10 Kj = indexdecl . getKeyPaths ;
11 Qj = indexdecl . getQualifierPaths ;
12 vj = indexdecl . getReturnValuePath ;
14 i f (Kq==Kj AND Qq==Qj AND vq==vj )
15 return j ; // Exactly matching index found
17 else i f ( Kj subset Kq AND Qj subset QQ)
18 subsetIndexes . add ( Kj ) ;
19 }
21 i f ( subsetIndexes . size ==1)
22 return subsetIndexes [ 1 ] ;
23 else
24 return sdg . findBestIndex ( subsetIndexes , q ) ;
25 }
Figure 5.9: Phase 1: Index Selection
2. Phase 2: Key Retrieval
In the second phase the key(s) are searched in the tree structure of the
index; like in relational indexes this is done in logarithmic time. If the key
is found the attached value is either the reference to the corresponding node
in the XML data (single-key index) or a search tree of a lower level (multi-key
index) so that further (recursive) key retrieval is performed. The references
are dereferenced to XML nodes and returned as the result of the query.
1 public XMLNOdeList evaluateQuery ( Index i , XPath q , XMLData t ) {
3 tree = i . getSearchTree ( ) ;
4 keys = q . getKeyValues ( ) ;
5 int ids = r e t r i e v a l ( tree , keys ) ;
7 nodes = new List ( ) ;
8 f o r a l l id in ids{
9 node = t . getXMLNodeByID ( id ) ;
10 nodes . add ( node ) ;
11 }
12 return nodes ;
13 }
17 private IDList r e t r i e v a l ( SearchTree tree , keys k ) {
19 i f ( isEmpty ( k ) ) {
20 i f ( tree is single node) return tree . id ;
21 else return getAllIds ( tree ) ;
22 }