url - Universität zu Lübeck

5.2. INDEX CREATION ALGORITHM 83
p has exactly one key path because it is a single-key query (see section 2.3.2).
• Second, for each key node we navigate to the corresponding return value
leading to a set of (key, return value) tuples. For each of these tuples we test
whether the conditions expressed by the qualifiers are fulfilled. If we have
no qualifiers this step can be omitted.
• The last step stores the remaining tuples in a new search tree dedicated to
executing queries of the shape of p.
The creation procedure of a multi-key index is a little more complex because
the keys of the different key paths are arranged in levels. Therefore, we use a
recursive algorithm that nests search trees for the key paths in different levels.
The (key, return value) tuples are stored in the last level. We present a Java
derived pseudocode of the creation algorithm for single-key and multi-key indexes
in Listing 5.1:
1 void createIndex (XMLNode context ,
2 SetOfKeyPaths Kp,
3 SetOfQualifierPaths Qp,
4 ReturnValuePath rp ,
5 SearchTree tree ) {
6 i f ( |Kp| == 1) { //Kp has only 1 key path
7 keyP = Kp[ 1 ] ; //get path to this keys
8 relKeyP = keyP−context ; // r e l a t i v e path to the keys from context node
9 relValP = rp−keyP ; // r e l a t i v e path to the return value
11 KEYS = eval ( context , relKeyP ) ; // r e t r i e v e the key elements
12 f o r a l l ( k in KEYS) {
13 val = eval ( k , relValPath ) ; // r e t r i e v e the return value
14 i f ( testQualifier ( val , Qp) ) { //t e s t q u a l i f i e r
15 int id = val . getId ( ) ; //get id of return node
16 tree . add ( k . getText ( ) , id ) ; //add key−value tuple
17 }
18 }
19 }
20 else{ //build a multikey index
21 keyP = Kp[ 1 ] ; //take f i r s t KeyPath
22 Kp = Kp \ Kp[ 1 ] ; //and remove i t from Kp
23 relKeyP = keyP−context ; // r e l a t i v e path to keys from context
25 KEYS = eval ( context , relKeyP ) ; // r e t r i e v e the key elements
26 f o r a l l ( k in KEYS)
27 SearchTree tree2 = new SearchTree ( ) ; // create empty search tree
29 createIndex ( k , Kp, Qp, rp , tree ) ; // recursive c a l l of method
31 tree . add ( k. getText ( ) , tree2 ) ; //nest the second tree
32 }
33 }
34 }
Figure 5.1: Pseudocode to create a single-key or multi-key index
The algorithm assumes that the path expressions to the keys, qualifiers, and the
return value have been extracted from a given XPath query. These path expressions
are extracted by the use of the functions key, qualifier, and value that are