url - Universität zu Lübeck

56 CHAPTER 4. INTRODUCTION TO RECENT APPROACHES IN XML INDEXING
sions; these approaches are itemized as hybrid indexes.
The selectivity on an index states whether it always covers the whole XML data
or is tunable for specific and user-defined fragments. A non-selective index has
to be updated whenever the original data is modified. A selective index consumes
less space and can be tuned for the typical usage of the database leading to less
update operations. A relational index is selective because it is defined upon a
table and a column.
Key-queries may return an element which differs from the key-element(s) that
is/are used for the value comparison. For instance, the general path expression
//item[quantity > x 1 ] returns item elements whereas the value used for the comparison
belongs to a quantity element. The majority of index approaches can
only return the indexed key-element leading to additional expenses for navigation
if the return element is different. For large paths between key and the return
value this may add significant costs for the query processor. Some approaches
like KeyX and the Refined Path from the Index Fabric are able to directly return
the requested element without further navigation in the XML data.
In order to explain and illustrate the different indexing approaches in a quickly
understandable manner we use some XML data taken from the XMark project
and generate a specific index for each approach to be evaluated. The sample data
consists of two items, one located in Asia and two in Europe. The items have
different child elements describing the properties of the item. Additionally, the
sample data contains two persons with their addresses. The textual representation
of the sample data is presented in figure 4.1.
1
2
3
4
5 Singapur
6 2
7 512 MB USB Stick
8 Money order
9 Cash
10
11
12
13
14 Hamburg
15 1
16 Beuys Sculpture
17
18
19 Paris
20 2
21 Louvre Tickets
22 Cash
23
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25