Anticipatory DTW for Efficient Similarity Search in Time Series Databases

Transcription

1 Anticipatory DTW for Efficient Similarity Search in Time Series Databases Ira Assent Marc Wichterich Ralph Krieger Hardy Kremer Thomas Seidl RWTH Aachen University, Germany Aalborg University, Denmark VLDB 2009 Lyon, France

2 Overview 1 Introduction 2 Dynamic Time Warping 3 Anticipatory pruning 4 Experiments 5 Conclusion Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 2 / 21

3 Time series similarity search Time series Sequence of time related values Stock data, sensor data, EEG measurements, climate data,... value (t i ) [mv] time series from EEG data set point in time (i) Similarity search Find time series with similar patterns over time Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 3 / 21

4 Dynamic Time Warping (DTW) Euclidean Distance Most widely used distance functions: Euclidean distance and Dynamic Time Warping Dynamic Time Warping allows scaling and stretching for better alignment, but computationally costly Euclidean Distance Dynamic Time Warping Figure: Euclidean distance (left) and Dynamic Time Warping (right) Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 4 / 21

5 DTW definition k-band DTW DTW ([s 1,..., s n ], [t 1,..., t m ]) = DTW ([s 1,..., s n 1 ], [t 1,..., t m 1 ]) dist band (s n, t m ) + min DTW ([s 1,..., s n ], [t 1,..., t m 1 ]) DTW ([s 1,..., s n 1 ], [t 1,..., t m ]) with dist band (s i, t j ) = { dist(s i, t j ) i j n m k else DTW (, ) = 0, DTW (x, ) =, DTW (, y) = Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 5 / 21

6 DTW Computation t j s i dist(s i, t j ) + min{c i-1,j-1,c i,j-1,c i-1,j } Figure: Cumulative warping matrix Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 6 / 21

7 Existing DTW algorithms DTW is computationally expensive Many approaches use multistep filter-and-refine architecture If filter lower bounds DTW lossless Different filters have been proposed and achieve substantial speed-ups query index (filter) candidates refinement database (exact) result Figure: Multistep filter-and-refine architecture Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 7 / 21

8 DTW properties for speed-up DTW is incremental. For any cumulative DTW matrix C = [c i,j ], the column minima are monotonically non-decreasing: min i=1,...,n {c i,x } min i=1,...,n {c i,y } for x < y. Existing approach: early stopping /early abandon compute DTW cumulative matrix after each filled column (band), check threshold for pruning Not as tight as it could be new anticipatory pruning tj si dist(s i,t j) + min{c i-1,j-1,c i,j-1,c i-1,j} Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 8 / 21

9 Anticpatory pruning query q data DTW filter 1 AP n candidatesap result anticipatory pruning AP 2 Figure: Anticipatory pruning in multistep filter-and-refine Multistep with anticipatory pruning Benefit from work already done in filter step Low overhead, substantial speed-up Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 9 / 21

10 General idea Anticipatory pruning: As in early stopping, compute DTW incrementally Additionally: re-use filter information to anticipate full DTW (no additional computation necessary!) Requires: filter for remainder of time series We characterize a class of filters to construct such anticipation: piecewise DTW property: reversible DTW estimate DTW estimate DTW estimate Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 10 / 21

11 Piecewise filter Piecewise DTW lower bound. A piecewise lower bounding filter for the DTW distance is a set f = {f 0,..., f m } with the following property: j = 0 : f j (s, t) = 0 j > 0 : f j (s, t) min DTW ([s 1,..., s i ], [t 1,..., t j ]) (i,j) band j Piecewise is the property of the filter that complements the incremental nature of DTW DTW estimate DTW estimate DTW estimate Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 11 / 21

12 DTW is reversible DTW is reversible. For any two time series [s 1,..., s n ] and [t 1,..., t m ] their DTW distance is the same as for the reversed time series: DTW ([s 1,..., s n ],[t 1,..., t m ]) = DTW ([s n,..., s 1 ],[t m,..., t 1 ]) Reversible is the DTW property that allows alteration of the time series direction between filter and DTW DTW estimate DTW estimate DTW estimate Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 12 / 21

13 Introduction Dynamic Time Warping Anticipatory pruning Experiments Conclusion [5, 3, 4, 5, 3, 4, 8, 3, 9, Anticipatory pruning Anticipatory Pruning Distance. q Given two time series s and t of length 9 n and m, a cumulative distance matrix C = [c i,j ], a piecewise lower bounding filter f for reversed time series, the j th anticipatory pruning is AP j (s, step t) j: 0 := 1 2 min 3 4 {c 5 6 i,j} + 7 f 8 m j (s 9 10, t ). min{}: 0 2 5i=1,...,n f : AP: DTW AP=min{} + f distance pruning threshold 10 f: anticipatory part 5 on inverted time series min{}: early stopping part 0 step j: Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 13 / 21

14 Example [5, 3, 4, 5, 3, 4, 8, 3, 9, 10] step j: min{}: f : AP: DTW 35 Figure: Anticipatory pruning example 30 q t [3, 8, 2, 0, 2, 6, 6, 4, 0, 2] AP=min{} + f Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 14 / 21 35

15 Anticipatory pruning is lossless Theorem: Anticipatory Pruning lower bounds DTW. Anticipatory pruning between two time series s, t with respect to a bandwidth k and a lower bounding filter f lower bounds the DTW: Proof sketch AP j (s, t) DTW (s, t) j {1,..., n} Anticipatory pruning: series of partial DTW paths plus lower bound estimate of the remainder from the previous filter step (1) For each possible step r, 1 r n, column minima lower bound the true path (2) DTW is reversible (3) Combine (1),(2): lower bound of DTW Lower bounding means: lossless pruning, i.e. speed-up + no loss of accuracy Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 15 / 21

16 Existing piecewise lower bounds Linearization LB Keogh : Euclidean distance to envelopes (upper, lower bound of segments) [Keogh, VLDB 2002; Zhu/Shasha, SIGMOD 2003] Corner boundaries LB Hybrid : piecewise corner-like shapes in the warping matrix through which every warping path has to pass [Zhou/Wong, ICDE 2007] Path Approximation FTW (Fast search method for Dynamic Time Warping): go from coarser DTW (less costly) to finer as needed: [Sakurai/Yoshikawa/Faloutsos, PODS 2005] are all piecewise lower bounds as required for anticipatory pruning (details in paper) Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 16 / 21

17 Experimental setup Synthetic and real world data sets SignLanguage: 1, 400 multivariate (11 attributes) of sign language finger tracking data [1], length 64 to 512 TRECVid: 650 to 2, 000 benchmark data [2] NEWSVid: 2, 000 to 8, 000 TV news recorded at 30 fps (20 attributes), length 64 to 2048 Random walk RW1/RW2: zero normalized time series of length 512 and of cardinality 10, 000 (1 to 50 attributes) 40 time series generated via RW time series generated via RW2 value (t i ) value (t i ) (a) RW1 t (i+1)j point = in time t ij + (i) N(0, 1) (b) RW2 t (i+1)j = tpoint ij + N(t in time ij (i) t (i 1)j, 1) [1] waleed/tml/data/ [2] Smeaton/Over/Kraaij, Evaluation campaigns and TRECVid, MIR 2006 Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 17 / 21

18 relative Improvement [percent] Introduction Dynamic Time Warping Anticipatory pruning Experiments Conclusion Experiments number of attributes AP_FTW ES_FTW AP_LBKeogh ES_LBKeogh AP_LBHybrid ES_LBHybrid Figure: Relative improvement (#calc.) for varying number of attributes on RW2 Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 18 / 21

19 relative Improvement [percent] Introduction Dynamic Time Warping Anticipatory pruning Experiments Conclusion Experiments AP_LBKeogh ES_LBKeogh AP_FTW ES_FTW AP_LBHybrid ES_LBHybrid k Figure: Efficiency improvement (#calc.) for varying DTW bandwidths on NEWSVid Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 19 / 21

20 average query time [s] log. scale average query time [s] log. scale average query time [s] log. scale Introduction Dynamic Time Warping Anticipatory pruning Experiments Conclusion Experiments LBKeogh ES_LBKeogh AP_LBKeogh reduced length FTW ES_FTW AP_FTW reduced length LBHybrid ES_LBHybrid AP_LBHybrid reduced length Figure: (log. scale) Absolute improvement (average query time), reduction, RW2 Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 20 / 21

21 Conclusion Anticipatory pruning Speed up DTW (Dynamic Time Warping) Widely used distance function for time series similarity search Our novel anticipatory pruning makes best use of A family of multistep filter-and-refine approaches Compute an estimated overall DTW distance from already available filter information: series of lower bounds of the DTW query q data DTW filter 1 AP n candidatesap result anticipatory pruning Experiments demonstrate substantially reduced runtime AP can be flexibly combined with existing and future DTW lower bounds AP is orthogonal to speed-up via indexing etc. AP 2 Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 21 / 21

22 Conclusion Anticipatory pruning Speed up DTW (Dynamic Time Warping) Widely used distance function for time series similarity search Our novel anticipatory pruning makes best use of A family of multistep filter-and-refine approaches Compute an estimated overall DTW distance from already available filter information: series of lower bounds of the DTW query q data DTW filter 1 AP n candidatesap result anticipatory pruning Experiments demonstrate substantially reduced runtime AP can be flexibly combined with existing and future DTW lower bounds AP is orthogonal to speed-up via indexing etc. AP 2 Thank you for your attention. Questions? Assent et al. Anticipatory DTW for Efficient Similarity Search in Time Series Databases 21 / 21