url - Universität zu Lübeck

5.3. KEYX BY EXAMPLES 87
and the value v of an index may be different. For instance, in p 2 the keys are the
values of the element author whereas the values are the corresponding inproceedings.
Of course, it is possible that the indexed key k is the returned value v of
a query. This concept avoids costly navigation from a key to the return value in
the XML data. This navigation is only performed once when creating the index or
adding new keys. The index structure for p 2 is shown in figure 5.4.
Index i p4
Key path: /dblp/inproceedings/author
Value path: /dblp/inproceedings
...
dblp
0
1
inproceeding
inproceeding
book
Dan Suciu
Mary F. Fernandez
2
8
13
author author title year ee author title year url author author author title year isbn
...... ......
...... ......
3 4 5 6 7
9 10 11 12
14 15 16 17 18
19
Figure 5.4: Established index i p2
Please note that this index is specific for queries p 2 indexing all authors of inproceedings.
The key ’Dan Suciu’ in the index structure references the inproceedings
of this author but not his book.
The path expressions of queries may also contain the self-or-descendant (//) and
wildcards (*) axis shown in the next example. Therefore, it is possible to keep
both the authors of books and the authors of articles in one index structure.
5.3.2 Multi-Key Indexes
If a query involves more than one key we need a multi-key index. For example,
the query p 7 = /dblp/ ∗ /title[../author. = ′ x ′ and ../year > y] looks for titles of any
publication (*) written by a given author and a year. This path expression contains
the two keys author and year.
A multi-key index in KeyX is constructed like in RDBMS: The index’s tree structure
is ordered by a first key (e.g. author), the values of this key are indexes
which are built upon the next key (year) and so on. Our approach does not combine
multiple keys and paths to one artificial key like the Index Fabric’s approach
does (see section 4.3.4). That is the reason why KeyX supports range queries easily.
As our tree implementation differentiates between texts and numbers it does
not have to normalize numeric values. Figure 5.5 shows the index for p 7 ; it is a
tree consisting of trees; the second tree’s values reference the XML data. Due to
the wildcard operator in this query both inproceedings and books are indexed.