1. Dynamic Queries over Mobile Objects
Iosif Lazaridis1
Kriengkrai Porkaew2
Sharad Mehrotra1
1 University of California, Irvine, USA
2 King Mongkut’s University of Technology
at Thonburi, Thailand
EDBT 2002, Prague, Czech Republic

2. Talk Outline Problem Motivation: Moving Observers
Preliminaries: Motion Representation and Indexing
Dynamic Queries : predictive Vs non-predictive
Evaluation Algorithms for Dynamic Queries
Experiments
Conclusions

3. Problem Motivation: Moving Observers

4. Problem Motivation: Moving Observersx y
observer
query
moving object x y x y
time

5. Preliminaries: Motion Representationx
O.x=f(t,)
Object O
max(O.x)
O.x=f(t,’)
min(O.x) tl th
time
Example Representation:
 = (x0, v)
O.x = x0 + v(t-tl) tl x0
slope v th

6. Preliminaries: Motion Indexingb e a d c g h f i j k a b c d e f g h i j k
time

7. Definitions: Snapshot and Dynamic Queriesx
time x Q0 Q4 Q3 Q2 Q1
Dynamic Query: Q0, Q1,…,Q4 c Q
xQh a
xQl b tQ
time
Snapshot Query: Q

8. Query Types: Predictive Dynamic Queries (PDQ)
User trajectory is known beforehand as a sequence of key snapshot queries, K0, K1,… K1 x
key snapshot K2 K3 K0
snapshot
time

9. Query Types: Non-Predictive Dynamic Queries (NPDQ)
P Q P x
Results retrieved in previous query P can
be useful in answering subsequent query Q if
P Q is significant
P and Q are temporally close
Objects are not too fast-moving

10. Comparison: Predictive Vs Non-Predictive Queries
Predictive queries are useful when
Trajectory is pre-planned (“tour mode”), or stable
Objects are returned in order of appearance along the known trajectory. Each object is retrieved once.
Non-predictive queries:
General case when the next query cannot be inferred
Objects that have been retrieved in the previous query are not retrieved again if they continue to appear in the next query
An interesting topic of future research:
Online adaptation to query behavior

11. Query Evaluation: Simple Approaches
General “Naïve” Approach
Ask each snapshot query separately
Disadvantage: redundant retrieval of objects; knowledge of trajectory (if any) is not used
Pre-calculate query results for PDQ
Evaluate the complete PDQ result set beforehand
Disadvantages
costly pre-computation
observer deviation?
updates?

12. Query Evaluation: Predictive Queries - Overview
tappear
tdisappear
For each tree node, or motion segment (starting from the root) we compute an interval [tappear, tdisappear] x a
getNext(tstart, tend) interface returns objects that will appear in view during [tstart, tend] in the order of their appearance time tappear
For a snapshot at time tq, repeated calls to getNext are made until an object with tappear>tq is returned
Returned objects are keyed in order of disappearance time tdisappear and are discarded from the query result as soon as tdisappear tq
tstart
tend
time

13. Query Evaluation: Predictive Queries - Example
tree node b
motion segment C C1 C2 d E e1 e2 f G h
time
tstart
tend

14. Query Evaluation: Predictive Queries - Motion Updates
R-Tree
Priority Queue A
b c 12
head
Before Update C f B
d e f
g h 3 c
0 1
2 3 4
5 6 7
8 9
10 11 4 D E F G H
After Update A n
b n c
head C f B
d f N
e m
g h n 3
0 1
5 7
2 3 4
6 12
8 9
10 11 c D F E M G H 4

15. Query Evaluation: Non-Predictive Queries
Ending time
Starting time Q P C D
TIME
SPACE y Q A P B x

16. Experiments: Data and Queries
Data and Index
2-d space of 100x100 space units
Each object updates its motion every ~1 time unit
Objects move with a velocity of ~1 space/time unit
Simulation runs for 100 time units
R-Tree with 4K page size and 0.5 fill factor is used (height=3)
Queries
1,000 trajectories. Snapshot query is issued every 0.1 time units
Varying overlap level between snapshots from 0 to 99.99%
Varying spatial range from 8x8 to 20x20

17. Experiments: Predictive Queries I/O

18. Experiments: Predictive Queries CPU

19. Experiments: Non-Predictive Queries I/O

20. Experiments: Non-Predictive Queries CPU

21. Conclusions
A Dynamic Query is a sequence of queries generated by a mobile observer
We proposed algorithms to evaluate Dynamic Queries for both the general (Non-Predictive) and a special (Predictive) case
Future work:
Generalizing Dynamic Queries
Adapting query processing to the behavior of the mobile observer