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142 CHAPTER 7. THE XML INDEX UPDATE PROBLEM
7.6.1 Update Algorithm
The first step of the algorithm checks if an index affection has happened or not
and calls the corresponding sub-routines. The intersection is analyzed for p and
all path expressions that define the established indexes. If the operations is an
insert or delete, the keys, qualifiers and the return value of each index must be
checked because these two operations change the structure and the content of
the XML data. A change operation changes only the content, therefore only the
keys have to be checked whether an existing value has been modified. The pseudo
code in listing 7.8 shows the first step:
1 f o r a l l i in indexes{ //check a l l indexes
2 K = i . getKeyPaths ( ) ; //extract the path expressions
3 Q = i . getQualiferPaths ( ) ; //that define an index
4 v = i . getReturnPath ( ) ;
6 i f ( type==insert | | type == delete ) {
8 i f ( type == insert )
9 execute o on XML data ; // modify the XML data
11 f o r a l l key in K{ //check a l l keys
12 i f intersection ( key , p ) !=ø
13 maintain ( type , i , p , ” keyAff ” ) ;
14 }
15 f o r a l l qual in Q{ //check a l l q u a l i f i e r s
16 i f intersection ( qual , p ) !=ø
17 maintain ( type , i , p , ” qualAff ” ) ;
18 }
19 i f intersection ( v , p ) !=ø //check the return value
20 maintain ( type , i , p, ” r e t A f f ” ) ;
22 i f ( type==delete )
23 execute o on XML data ; // modify the XML data
24 }
26 else i f type==change ) {
27 f o r a l l key in K{ //check a l l keys
28 i f intersection ( key , p ) !=ø
29 maintain ( type , i , p , , ” keyAff ” ) ;
30 }
31 execute o on XML data ; // modify the XML data
32 }
33 }
Figure 7.8: Pseudocode of the update algorithm
Besides maintaining the indexes the modifying database operation must be performed
on the underlying XML data as well. If o is a delete or a change this
operation is performed after maintaining the indexes because we need access to
the information that is going to be deleted/changed. For an insert o is performed
before maintaining the indexes because we need the context of the new node.