With the increasing use of RDF graphs, storing and querying such data using SPARQL remains a critical problem. Current mainstream solutions rely on cloud-based data management architectures, but often suffer from performance bottlenecks in environments with limited bandwidth or high system load. To address this issue, this paper explores for the first time the integration of edge computing to move graph data storage and processing to edge environments, thereby improving query performance. This approach requires offloading query processing to edge servers, which involves addressing two challenges: data localization and network scheduling. First, the data localization challenge lies in computing the subgraphs maintained on edge servers to quickly identify the servers that can handle specific queries. To address this challenge, we introduce a new concept of pattern-induced subgraphs. Second, the network scheduling challenge involves efficiently assigning queries to edge and cloud servers to optimize overall system performance. We tackle this by constructing a overall system model that jointly captures data distribution, query characteristics, network communication, and computational resources. Accordingly, we further propose a joint formulation of query assignment and computational resource allocation, modeling it as a Mixed Integer Nonlinear Programming (MINLP) problem and solve this problem using a modified branch-and-bound algorithm. Experimental results on real datasets under a real cloud platform demonstrate that our proposed method outperforms the state-of-the-art baseline methods in terms of efficiency. The codes are available on GitHub

Unsupervised graph alignment finds the node correspondence between a pair of attributed graphs by only exploiting graph structure and node features. One category of recent studies first computes the node representation and then matches nodes with the largest embedding-based similarity, while the other category reduces the problem to optimal transport (OT) via Gromov-Wasserstein learning. However, it remains largely unexplored in the model expressiveness, as well as how theoretical expressivity impacts prediction accuracy. We investigate the model expressiveness from two aspects. First, we characterize the model'sdiscriminative powerin distinguishing matched and unmatched node pairs across two graphs. Second, we study the model's capability of guaranteeingnode matching propertiessuch as one-to-one matching and mutual alignment. Motivated by our theoretical analysis, we put forward a hybrid approach namedCombAlignwith stronger expressive power. Specifically, we enable cross-dimensional feature interaction for OT-based learning and propose an embedding-based method inspired by the Weisfeiler-Lehman test. We also apply non-uniform marginals obtained from the embedding-based modules to OT as priors for more expressiveness. Based on that, we propose a traditional algorithm-based refinement, which combines our OT and embedding-based predictions using the ensemble learning strategy and reduces the problem to maximum weight matching. With carefully designed edge weights, we ensure these matching properties and further enhance prediction accuracy. By extensive experiments, we demonstrate a significant improvement of 14.5% in alignment accuracy compared to state-of-the-art approaches and confirm the soundness of our theoretical analysis.

Given a query graph, top-k subgraph matching finds up to$k$matches in a data graph with the highest scores according to a user-defined scoring function. It has wide applications across many fields, including knowledge graphs and social networks. Due to the enormous search space, existing methods are not efficient enough on large graphs. In this paper, we proposePTAB, an efficient serial algorithm for top-k subgraph matching. It traverses an efficiently pruned search space by topology-aware subspace score upper bounds computed from a novel hop index, which stores the range of node properties in a constrained multi-hop neighborhood of each node. Additionally,PTABintegrates a cost-aware root selection strategy, which chooses query nodes leading to a search process that utilizes the pruning power of the hop index as much as possible. Furthermore, we use a novel edge-cut strategy to handle general query graphs with cycles. We also extendPTABto a parallel version,PTABP, which further improves performance via multi-threading and enhanced pruning techniques. Experimental results on real and synthetic datasets demonstrate that our methods outperform existing methods.

With the advent of graph data, graph databases have garnered significant research interest and efforts in recent years, especially with respect to graph query processing. There have been a vast suite of methods for efficient graph query processing, especially for the core graph query constructs, regular path queries (RPQs) and subgraph matching queries (SMQs). In the meantime, there is an observable divide among these methods as well as confusion between them and their relational counterparts. We thus propose this tutorial to provide a unified narrative for graph query processing, so as to bridge the gap between existent lines of work and offer a comprehensive view of the query processing workflow in graph databases.

The proliferation of RDF datasets has resulted in studies focusing on optimizing SPARQL query processing. Most existing work focuses on basic graph patterns (BGPs) and ignores other vital operators in SPARQL, such as UNION and OPTIONAL. SPARQL queries with these operators, which we abbreviate as SPARQL-UO, pose serious query planning challenges. In this paper, we propose techniques for optimizing SPARQL-UO queries using BGP execution as a building block, based on a novel BGP-based Evaluation (BE)-Tree representation of query plans. On top of this, we propose a series of cost-driven BE-tree transformations to generate more efficient plans by reducing the search space and intermediate result sizes, and a candidate pruning technique that further enhances efficiency at query time. Experiments confirm that our method outperforms the state-of-the-art by orders of magnitude.

Subgraph matching is a fundamental task in graph analysis systems. In real-world applications, graph query engines often need to process a large number of subgraph matching queries, many of which share common substructures. Materializing the results of these shared subqueries as view patterns can enable computation reuse and significantly improve query efficiency. However, existing view materialization techniques suffer from either high memory usage or limited acceleration benefits.This paper introduces MAVIS, a novel view-based subgraph matching algorithm. MAVIS partitions view patterns into connected subgraphs called super-nodes and performs super-node-oriented materialization to balance memory consumption and processing speed. To further improve efficiency, it proposes a tree-based super-node partitioning method that avoids generating invalid candidates during materialization. Additionally, a customized query answering algorithm is designed to leverage the materialized views for faster query execution.Extensive experiments on real-world datasets demonstrate that MAVIS achieves a superior trade-off between memory usage and acceleration, and it outperforms existing approaches.

Subgraph query can be applied in various scenarios, such as fraud detection and cyberattack pattern analysis. However, computing subgraph queries usually traverses a huge search space. Many efforts have been made to reduce this search space. The size of the answer set can be exponential, providing a substantial lower bound for the search space. Additionally, different answers may overlap, and a single vertex can occur multiple times in different matches. In this paper, we propose a new problem to compute the match cover of a subgraph query. We define the match cover as a subset of answers such that the vertices included are exactly the same as those in the entire set. There can be more than one match covers, however, we only return one, as long as we can avoid the huge overhead of searching the entire set. It is inefficient to apply traditional subgraph query methods for computing match cover. Specifically, existing methods do not prune partial matches that could grow into full matches. For match cover computation, if the vertices in those full matches are already included in previously found matches, continuing the computation over such partial matches is a waste of time. We propose a new framework, called MatCo, to compute the match cover. In MatCo, we design a new data structure, called local candidate space, to determine whether the future search scopes of partial matches have been covered. We can easily maintain local candidate space and efficiently conduct the determination. We also reduce some Cartesian products, which are inevitable in existing methods, into linear enumerations, which significantly improves performance. Extensive experiments over various datasets confirm that our method outperforms comparative ones by 1~3 orders of magnitude. Efficiently computing the minimum match cover could be an interesting future work.

Multi-way join, which refers to the join operation among multiple tables, is widely used in database systems. With the development of the Internet and social networks, a new variant of the multi-way join query has emerged, requiring continuous monitoring of the query results as the database is updated. This variant is called continuous multi-way join. The join order of continuous multi-way join significantly impacts the operation's cost. However, existing methods for continuous multi-way join order selection are heuristic, which may fail to select the most efficient orders. On the other hand, the high-cost order computation will become a system bottleneck if we directly transfer join order selection algorithms for static multi-way join to the dynamic setting. In this paper, we propose a new A daptive J oin O rder S election algorithm for the C ontinuous multi-way join queries named AJOSC. It uses dynamic programming to find the optimal join order with a new cost model specifically designed for continuous multi-way join. We further propose a lower-bound-based incremental re-optimization algorithm to restrict the search space and recompute the join order with low cost when data distribution changes. Experimental results show that AJOSC is up to two orders of magnitude faster than the state-of-the-art methods.

Subgraph matching, a cornerstone of graph analytics, critically suffers from redundant computations during the search process. Existing methods primarily target identical computations redundant operations that are localized to individual query vertices but fail to address similar redundancies that recur across multiple query vertices. In this paper, we present a novel algorithm, called FiPE, that accelerates subgraph matching through Fine-grained and Powerful Equivalences. FiPE redefines redundancy elimination by shifting the optimization granularity from isolated vertices to vertex pairs and multiple vertex patterns. It introduces vertex-pair equivalence to cluster candidate pairs with isomorphic neighbor structures, even if their individual vertices differ, enabling pruning of similar computations between these vertex pairs. FiPE proposes group equivalence to defer equivalence checks to later search depths, capturing potential redundancies incrementally. To fully exploit the advantages of the equivalence, we introduce two optimization techniques: a matching order generation method to reduce the overall search space and an efficient conflict resolution mechanism to avoid two query vertices being mapped to the same data vertex. Experiments on real-world graphs highlight the superiority of FiPE. FiPE achieves a speedup of 2 to 3 orders of magnitude on various graphs under the EPS (embeddings per second) metric.

Graph databases play a pivotal role in the FinTech industry. However, existing graph benchmarks fail to capture the unique characteristics of financial datasets and workloads, rendering them inadequate for evaluating graph databases in financial scenarios. This paper presents the LDBC Financial Benchmark (FinBench) Transaction Workload, a novel benchmark that adopts a choke point-driven design methodology, emphasizing performance bottlenecks, and incorporates distinct features such as dataset skewness, edge multiplicity, temporal window filtering, recursive path filtering, read-write query patterns, and truncation on hub vertices. Key contributions include a scalable data generator that synthesizes datasets with financial-specific features, a parameter generator that leverages bucketed data statistics for runtime consistency across queries, and a scalable benchmark driver that biases query execution by time windows. Experimental evaluations on graph databases demonstrate the benchmark's capability to reveal novel choke points and provide insights into system performance in financial scenarios.

Given a large graph$G$, a subgraph query$Q$finds the set of all subgraphs of$G$that satisfy certain conditions specified by$Q$. Examples of subgraph queries including finding a community containing designated members to organize an event, and subgraph matching. To overcome the weakness of existing graph-parallel systems that underutilize CPU cores when finding subgraphs, our prior system, G-thinker, was proposed that adopts a novel think-like-a-task (TLAT) parallel programming model. However, G-thinker targets offline analytics and cannot support interactive online querying where users continually submit subgraph queries with different query contents. The challenges here are (i) how to maintain fairness that queries are answered in the order that they are received: a later query is processed only if earlier queries cannot saturate the available computation resources; (ii) how to track the progress of active queries (each with many tasks under computation) so that users can be timely notified as soon as a query completes; and (iii) how to maintain memory boundedness and high task concurrency as in G-thinker. In this article, we propose a novel TLAT programming framework, called G-thinkerQ, for answering online subgraph queries. G-thinkerQ inherits the memory boundedness and high task concurrency of G-thinker by organizing the tasks of each query using a “task capsule” structure, and designs a novel task-capsule list is to ensure fairness among queries. A novel lineage-based mechanism is also designed to keep track of when the last task of a query is completed. Parallel counterparts of the state-of-the-art algorithms for 4 recent advanced subgraph queries are implemented on G-thinkerQ to demonstrate its CPU-scalability.

In-context learning (ICL) empowers large pre-trained language models (PLMs) to predict outcomes for unseen inputs without parameter updates. However, the efficacy of ICL heavily relies on the choice of demonstration examples. Randomly selecting from the training set frequently leads to inconsistent performance. Addressing this challenge, this study takes a novel approach by focusing on training data valuation through causal inference. Specifically, we introduce the concept of average marginal effect (AME) to quantify the contribution of individual training samples to ICL performance, encompassing both its generalization and robustness. Drawing inspiration from multiple treatment effects and randomized experiments, we initially sample diverse training subsets to construct prompts and evaluate the ICL performance based on these prompts. Subsequently, we employ Elastic Net regression to collectively estimate the AME values for all training data, considering subset compositions and inference performance. Ultimately, we prioritize samples with the highest values to prompt the inference of the test data. Across various tasks and with seven PLMs ranging in size from 0.8B to 33B, our approach consistently achieves state-of-the-art performance. Particularly, it outperforms Vanilla ICL and the best-performing baseline by an average of 14.1% and 5.2%, respectively. Moreover, prioritizing the most valuable samples for prompting leads to a significant enhancement in performance stability and robustness across various learning scenarios. Impressively, the valuable samples exhibit transferability across diverse PLMs and generalize well to out-of-distribution tasks.

Given a query graph, top-k subgraph matching finds up to k matches in a data graph with the highest scores according to a user-defined scoring function. It has wide applications across many fields, including knowledge graphs and social networks. Due to the enormous search space, existing methods are not efficient enough on large graphs. In this paper, we propose PTAB, an efficient algorithm for top-k subgraph matching. It traverses an efficiently pruned search space by topology-aware sub-space score upper bounds computed from a novel hop index, which stores the range of node properties in a constrained multi-hop neighborhood of each node. Additionally, PTAB integrates a cost-aware root selection strategy, which chooses query nodes leading to a search process that utilizes the pruning power of the hop index as much as possible. Furthermore, we use a novel edge-cut strategy to handle general query graphs with cycles. Experimental results on real and synthetic datasets demonstrate that our method outperforms existing methods.

Variable-length path queries are essential for finding paths in a graph that adhere to a specified length constraint, utilizing only edges with labels from a restricted subset of the edge labels. These queries play a crucial role in graph analytics and are supported by practical graph query languages like Cypher in property graph systems and SPARQL 1.1 in RDF graph systems. In this paper, we present a novel solution for efficient evaluation of variable-length path queries, based on worst-case optimal joins. Our solution's core relies on a jumping-like worst-case optimal join technique, allowing us to select a query vertex order that differs completely from existing graph systems based on worst-case optimal joins. Furthermore, we introduce a cost-based dynamic programming optimizer that combines traditional and jumping-like worst-case optimal join techniques. We also propose an optimization technique to leverage intra-query parallelism during query evaluation. Through extensive experiments conducted on numerous synthetic and real RDF and property graphs, we demonstrate that the proposed technique achieves excellent performance.

In this study, we address the problem of unnecessary computations in traditional continuous subgraph matching (CSM) frameworks due to premature expansions of the search space in dynamic graphs. Traditional CSM frameworks expand small partial matches according to a specific matching order until the final results are obtained. This extension involves two sequential steps: computing candidate vertices for an unmapped query vertex and expanding the search space using these candidate data. However, this long-established search model has a potential flaw, as premature expansions of the search space can lead to unnecessary computations. To address this issue, we introduce a novel search process, NewSP. Unlike traditional methods, NewSP emphasizes operations rather than extensions, incorporating a unique feature of postponing expansion at the operation level. This approach prevents premature expansions without compromising the initial pruning power of the selected matching order. Furthermore, NewSP allows for multiple consecutive expansions, paving the way for a multi-expansion strategy for further optimization. Our model also enables the implementation of cache strategies for candidate set reuse, as it does not necessitate immediate expansion of a candidate set once identified. To improve performance, we propose an adaptive index filtering strategy independent of the specific index used. Comprehensive experiments demonstrate that our method improves by up to two to three orders of magnitude compared to traditional algorithms. A case study showed that NewSP can accelerate the majority of subgraph matching algorithms.

A regular path query (RPQ) returns node pairs connected by a path whose edge label sequence satisfies the given regular expression. Given a workload of RPQs, selecting the shared subqueries as materialized views to precompute offline can speed up the online processing. Since the available memory is limited, we define the materialized view selection (MVS) problem for RPQs as minimizing the total workload query cost within a memory budget. To tackle the problem's NP-hardness, we design an efficient MVS algorithm based on heuristics. To prevent redundancies in the selected views, we devise the AND-OR directed acyclic graph with closure (AODC) as the multi-RPQ query plan representation for the workload, which encodes the relations between subqueries. In addition to detecting view redundancy, the AODC also incrementally updates itself during view selection. To support query planning, we design a scalable cost and cardinality estimation scheme for full-fledged RPQs, including Kleene closures. Our method, when applied to the Wikidata Query Logs, shows a 9.73× speedup in the total query processing time compared to ad-hoc processing, using the views it selects.

Regular path query (RPQ) is a basic operation for graph data analysis, and persistent RPQ in streaming graphs is a new-emerging research topic. In this paper, we propose a novel algorithm for persistent RPQ in streaming graphs, named LM-SRPQ. It solves persistent RPQ with a combination of intermediate result materialization and real-time graph traversal. Compared to prior art, it merges redundant storage and computation, achieving higher memory and time efficiency. We carry out extensive experiments with both real-world and synthetic streaming graphs to evaluate its performance. Experiment results confirm its superiority compared to prior art in both memory and time efficiency.

Dynamic graph management (DGM) systems are designed to effectively handle changing graph data, which is a fundamental problem for many graph-based applications. Recently, researchers have designed GPU-based solutions for DGM and its downstream applications, thanks to GPUs' massive parallelism power. However, there is a lack of universal models that summarize the features and design principles of GPU-accelerated DGM systems. Additionally, existing studies test GPU-based DGM systems without unified metrics and workloads. Under this circumstance, we propose a conceptual model for GPU-accelerated DGM to demonstrate a DGM system's components, key primitives, and optimization choices. Next, we evaluate six representative systems, testing their update and query performance with unified metrics and workloads of different algorithmic behaviors. We also extend existing systems to seek insight to fill the current research gap in multi-GPU support, concurrency control, resource utilization, and so on. Our evaluation yielded new insights on the pros and cons of different systems: (1) Hashing-based systems perform best for graph updates but may not be suitable for all applications. (2) Finding a system that fits all workloads is challenging, and hybrid data storage may be a solution. (3) To select the most suitable DGM system for a specific workload, it is essential to consider hardware-related metrics. Finally, we provide recommendations and suggestions for future studies based on our experimental results and observations.

Partitioning a large graph into smaller subgraphs by minimizing the number of cutting vertices and edges, namely cut size or replication factor, plays a crucial role in distributed graph processing tasks. However, many prior works have primarily focused on optimizing the cut size by considering only vertex balance or edge balance, leading to significant workload imbalance and consequently hindering the performance of downstream tasks. Therefore, in this paper, we address the dual-balanced graph partition problem that minimizes the cut size while simultaneously guaranteeing both vertex and edge balance. We propose a lightweight effective two-phase framework, namely fine-grained splitting and merging (FSM), which decomposes the graph into more and smaller partitions and then merges them. FSM offers the flexibility of integrating with various state-of-the-art single-balanced techniques. We develop two efficient algorithms Fast Merging and Precise Merging to enable trade-offs between computational efficiency and partitioning quality. Experimental results on large real-world graphs demonstrate that FSM achieves state-of-the-art cut size while maintaining dual balance. The runtime for downstream tasks PageRank, connected component, and diameter estimation, can be reduced by a large proportion, up to 9.43%, 11.35%, and 17.94%, respectively.

Map matching of sparse vehicle trajectories is a fundamental problem in location-based services, such as traffic flow analysis and vehicle routing. Existing literature mainly relies on sequence-to-sequence (Seq2Seq) models to capture theintra-trajectory correlationof an input trajectory and to sequentially predict the matched road segments. Due to the limited expressive capability of sequential models, these methods fall short of extractinginter-trajectoryandtrajectory-road correlationsas well ascorrelation between road segments. We presentGraphMM, a graph-based approach that explicitly utilizes all aforementioned correlations. Our model exploits the graph nature of map matching and incorporates graph neural networks and conditional models to leverage both road and trajectory graph topology, while manages to align road segments and trajectories in latent space. We formally analyze the expressive power of our model in capturing various correlations and propose efficient algorithms for model training and inference. In particular, our optimization techniques dramatically reduce the computational complexity, making our model feasible on datasets with thousands of road segments. Extensive experiments show that our model significantly enhances prediction accuracy, while improving training and inference efficiency by up to an order of magnitude over both the industrial implementation of the hidden Markov model and state-of-the-art Seq2Seq-based methods.

We propose to design vertex encoding for determinations of no-result edge queries that should not be executed. Edge query is one of the core operations in mainstream graph databases, which is to retrieve edges connecting two given vertices. Real-world graphs may be too large to be stored in memory and frequently accessing edge data on disk usually incurs much overhead. The average degree of real-world graph tends to be much less than the vertex number, and edges may not exist in most pairs of vertices. Efficiently avoiding no-result edge query executions will certainly improve the performance of graph database. In this paper, we propose a new and important problem for determining no-result edge queries: vertex encoding for edge nonexistence determination (VEND, for short). We build a low dimensional vertex encoding for all vertices, and we can efficiently determine most vertex pairs that are connected by no edges just with their corresponding codes. The encoding can be efficiently adjusted when data updates happen. With VEND, we can utilize in-memory efficient operations to filter no-result disk accesses for edge query. We also design SIMD-oriented compression optimizations to further improve performance. Extensive experiments on real-world datasets confirm the effectiveness of our solution.

As a fundamental subtask of Knowledge Base Question Answering (KBQA), Relation Detection (KBQA-RD) plays a crucial role to detect the KB relations between entities or variables in natural language questions. It remains, however, a challenging task, particularly for significant large-scale relations and in the presence of easily confused relations. Recent state-of-the-art methods not only struggle with such scenarios, but often take into account only one facet and fail to incorporate the subtle discrepancy among the relations. In this paper, we propose a simple and efficient three-stage framework to exploit the coarse-to-fine paradigm. Specifically, we employ a natural clustering over all KB relations and perform a coarse-to-fine relation recognition process based on the relation clustering. In this way, our framework (i.e., CORD) refines the detection of relations, so as to scale well with large-scale relations. Experiments on both single-relation (i.e., SimpleQuestions (SQ)) and multi-relation (i.e., WebQSP (WQ)) benchmarks show that CORD not only achieves the outstanding relation detection performance in KBQA-RD subtask; but more importantly, further improves the accuracy of KBQA systems.

Graph pattern mining (GPM) is getting increasingly important recently. There are many parallel frameworks for GPM, many of which suffer from performance. GPU is a powerful option for graph processing, which has excellent potential for performance improvement; however, parallel GPM algorithms produce a large number of intermediate results, limiting GPM implementations on GPU.In this paper, we present GAMMA, an out-of-core GPM framework on GPU, and it makes full use of host memory to process large graphs. Specifically, GAMMA adopts a self-adaptive implicit host memory access manner to achieve high bandwidth, which is transparent to users. GAMMA provides flexible and effective interfaces for users to build their algorithms. We also propose several optimizations over primitives provided by GAMMA in the out-of-core GPU system. Experimental results show that GAMMA has scalability advantages in graph size over the state-of-the-art by an order of magnitude, and is also faster than existing GPM systems.

We propose to design vertex encoding for determinations of no-result edge queries that should not be executed. Edge query is one of the core operations in mainstream graph databases, which is to retrieve the corresponding edges connecting two given vertices. Real-world graphs may be too large to be stored in memory and frequently accessing edge data on disk usually incurs much overhead. Average degree of real-world graph tends to be much less than the vertex number, and edges may not exist in most pairs of vertices. Efficiently avoiding no-result edge query executions will certainly improve performance of graph database. In this paper, we propose a new and important problem for determining no-result edge queries: vertex encoding for edge nonexistence determination (VEND, for short). We build a low dimensional vertex encoding for all vertices, and we can efficiently determine most vertex pairs that are connected by no edges just with their corresponding codes. With VEND, we can utilize in-memory efficient operations to filter no-result disk accesses for edge query. We also design maintenance algorithms for the proposed solution when data updates happen. Extensive experiments on many real-world datasets confirm the ability of our solution on determining a quite high proportion of non-edge vertex pairs, as well as the acceleration for edge queries.

Reachability is a fundamental graph operator. State-of-the-art index-based reachability processing frameworks can efficiently handle static graphs, but the recent advent of dynamic graph data poses new challenges. To address these challenges, we propose an index-free, community-aware (IFCA) reachability processing framework inspired by efficient Personalized PageRank approximation algorithms, which identifies community structures on-the-fly to accelerate query processing. On top of it, we devise a community contraction technique to bridge the gap between vertices in distinct communities, and a cost-based strategy selection procedure to efficiently handle the resulting reduced graph. We conduct experiments with realistic query workloads over large-scale real dynamic graphs, showing our approach’s superior efficiency compared with index-based and index-free state-of-the-art methods.

Subgraph isomorphism plays a significant role in many applications, such as social networks and bioinformatics. However, due to the inherent NP-hardness, it becomes challenging to compute matches efficiently in large real-world graphs. Many researchers have attempted to solve this problem with the help of new hardware. Nevertheless, most of them focus on GPU. Due to the dataflow feature and burst I/O optimization, FPGA is a potential competitor to speed up subgraph isomorphism. However, there are very few subgraph matching algorithms on FPGA. In this paper, we present an efficient FPGA-friendly Subgraph Isomorphism algorithm FASI, designed on CPU- FPGA heterogeneous platform which leverages FPGA's features. Unlike the existing FPGA-based method FAST, we adopt the worst-case-optimal-join-based pipeline design. First, we propose an FPGA-friendly data structure LPCSR for efficient access to neighbor lists. Second, we offer a joint parallelized pipeline strategy to accelerate matching process. Third, we propose a memory coalescing mechanism and a space-saving pre-allocated write back strategy. Our experiments on both synthetic and real graphs show that FASI outperforms other state-of-the-art subgraph matching algorithms on CPU, GPU and FPGA.

Recent years have witnessed the transition from sentence-level to document-level in relation extraction (RE), with new formulation, new methods and new insights. Yet, the fundamental concept, mention, is not well-considered and well-defined. Current datasets usually use automatically-detected named entities as mentions, which leads to the missing reference problem. We show that such phenomenon hinders models' reasoning abilities. To address it, we propose to incorporate coreferences (e.g. pronouns and common nouns) into mentions, based on which we refine and re-annotate the widely-used DocRED benchmark as R-DocRED. We evaluate various methods and conduct thorough experiments to demonstrate the efficacy of our formula. Specifically, the results indicate that incorporating coreferences helps reduce the long-term dependencies, further improving models' robustness and generalization under adversarial and low-resource settings. The new dataset is made publicly available for future research.

SimRank-based similarity joins, which mainly include threshold-based and top- k similarity joins, are important types of all-pair SimRank queries. Although a line of related algorithms have been proposed recently, they still fall short of providing approximation guarantee and suffer from scalability issues on medium and large graphs. Meanwhile, we also lack an extensive analysis of existing techniques in terms of accuracy and efficiency. Motivated by these challenges, we first conduct detailed analysis of state-of-the-art algorithms and provide additional theoretical results. Second, to address the limitations of existing techniques, we propose simple yet effective algorithm frameworks for both queries to theoretically guarantee the approximation bound, and present a more efficient all-pair algorithm inspired by randomized local push of Personalized PageRank. Next, we analyze the algorithmic complexity of threshold-based and top- k similarity joins by leveraging a reasonable assumption of SimRank distribution. Through extensive experiments, we find that our proposed methods far exceed existing ones with respect to query efficiency, approximation guarantee and practical accuracy, while our theoretical analysis nicely matches the empirical study.

A graph stream is a continuous sequence of data items, in which each item indicates an edge, including its two endpoints and edge weight. It forms a dynamic graph that changes with every item. Graph streams play important roles in cyber security, social networks, cloud troubleshooting systems and more. Due to the vast volume and high update speed of graph streams, traditional data structures for graph storage such as the adjacency matrix and the adjacency list are no longer sufficient. However, prior art of graph stream summarization either supports limited kinds of queries or suffers from poor accuracy of query results. In this paper, we propose a novelGraphStreamSketch (GSS for short) to summarize the graph streams, which has linear space cost$O(|E|)$(E is the edge set of the graph) and high update speed, and supports most kinds of queries over graph streams with controllable errors. Experimental results show that our solution is up to 142 times faster than the adjacency list when processing updates in graph streams, and its memory consumption is as small as$30\%$of the adjacency list. Though error is introduced as a trade off in our solution, both theoretical analysis and experiment results confirm that such error is small and controllable. The relative error is below$10^{-2}$in edge weight query, and the precision is above$90\%$is 1-hop precursor/successor queries.

As a powerful expression of human knowledge in a structural form, knowledge graph (KG) has drawn great attention from both the academia and the industry and a large number of construction and application technologies have been proposed. Large-scale knowledge graphs such as DBpedia, YAGO and Wikidata are published and widely used in various tasks. However, most of them are far from perfect and have many quality issues. For example, they may contain inaccurate or outdated entries and do not cover enough facts, which limits their credibility and further utility. Data quality has a long research history in the field of traditional relational data and recently attracts more knowledge graph experts. In this paper, we provide a systematic and comprehensive review of the quality management on knowledge graphs, covering overall research topics about not only quality issues, dimentions and metrics, but also quality management processes from quality assessment and error detection, to error correction and KG completion. We categorize existing works in terms of target goals and used methods for better understanding. In the end, we discuss some key issues and possible directions on knowledge graph quality management for further research.

Due to the inherent hardness of subgraph isomorphism, the performance is often a bottleneck in various real-world applications. We address this by designing an efficient subgraph isomorphism algorithm leveraging features of GPU architecture. Existing GPU-based solutions adopt two-step output scheme, performing the same join twice in order to write intermediate results concurrently. They also lack GPU architecture-aware optimizations that allow scaling to large graphs. In this article, we propose aScalableGPU-friendlysubgraphisomorphism algorithm,SGSI. SGSI incorporates aPrealloc-Combinestrategy based on the vertex-oriented framework, which avoids joining-twice in existing solutions. It uses a GPU-friendly data structure (calledPCSR) to represent an edge-labeled graph. We also study fine-grained load balance strategies and discuss how to handle enormous graphs that cannot be resident in GPU memory. A partition-based pipeline framework is proposed. Extensive experiments on both synthetic and real graphs show that SGSI outperforms the state-of-the-art algorithms by up to several orders of magnitude and has a good scalability with graph size scaling to billions of edges.

There is a growing interest to offload dynamic graph computation to GPU and resort to its high parallel processing ability and larger memory bandwidths compared with CPUs. The existing GPU graph systems usually use compressed sparse row (CSR) as the de-facto structure. However, CSR has a critical weakness for dynamic change due to the large overhead of re-balance process after update. GPMA+ is a state-of-art dynamic PMA-based structure that uses PMA structure and segment-oriented parallel update procedure to address the dynamic weakness of CSR, but it still has a bottleneck on the array expansion. In this paper, we propose an leveled structure (called LPMA) instead of continue array to retain low time complexity and high parallel update and lift the expansion bottleneck of GPMA+. More specifically, we propose a series of optimization techniques, including bottom-up update, top-down update and on-demand hybrid update strategies as well as consistence-guaranteed parallel processing for update-query mixed workloads. We theoretically analyze the benefits of LPMA compared in terms of re-balance cost during updates. Extensive experiments on four large real-life graphs prove the superiority of LPMA compared with the-state-of-arts.

Graph pattern mining (GPM) is getting increasingly important in recent years. Many graph pattern mining frameworks try to use universal primitives to deal with various graph pattern mining tasks. However, most of them suffer from unsatisfactory performance because of the exponential complexity of GPM. GPU is a new hardware with great parallelism, and many graph algorithms have achieved significant performance improvements on GPU. In this demo, we propose a graph pattern mining framework on GPU, called GAMMA. GAMMA proposes effective and flexible interfaces for users to implement their mining tasks conveniently. GPM has extensive intermediate results in parallel environments. We make full use of host memory to deal with large-scale graphs and extensive intermediate results. We also present several optimizations to process large graphs. GAMMA has great scalability and performance advantages compared with state-of-the-art graph mining works in experiments.

Query optimization is an especially challenging problem in graph databases due to its wide plan space and the difficulty in gathering statistics. In this demonstration, we propose a new cost-based query optimizer called gCBO for graph databases and implement it in a specific graph database (i.e., gStore). To tackle the aforementioned challenges, gCBO employs a hybrid plan enumerator based on dynamic programming, cost models that capture the characteristics of different types of joins, and a sampling-based cardinality estimation strategy that gathers the necessary statistics on-the-fly. What is more, to absorb the experience of users, we build an interactive component for gCBO, which allows users to receive the optimized execution plans with detailed information and generate their own plans for execution.

Real-world graphs exhibit many interesting properties that differentiate them from random graphs, which have been extensively studied for the past decades. For various proposed generative models, a majority of them build the graph by sequentially adding each node and the attached edges. However, the growth of many real-world graphs, such as social networks, is naturally modeled by the sequential insertion of edges. Unfortunately, to the best of our knowledge, no generative model has been proposed to reveal this process. We propose the first sequence-of-edges model, denoted as temporal preferential attachment (TPA). It relies on preferential attachment (PA), one of the most influential mechanisms to generate scale-free graphs, and takes time-decay effect and node fitness into consideration. Empirical analysis demonstrates that our model preserves several key properties of the real-world graphs, including both the properties observed from the snapshot graphs (e.g., power-law distribution) and temporal properties observed from the graph generation process (e.g., shrinking diameter). Meanwhile, our model is sufficiently general to accommodate several forms of time decay and fitness distributions. Then, we design two efficient algorithms that generate TPA graphs with billions of edges in several minutes.

Scaling-out RDF processing to deal with graph size usually requires partitioning the RDF graph. Typical partitioning approaches minimize edge-cuts or vertex-cuts. In this paper we argue that these approaches do not avoid or reduce joins between different partitions (i.e., inter-partition join), and propose an approach based on minimizing the number of distinct crossing properties, which we call Minimum Property-Cut (MPC). This approach enables more queries to be independently evaluated without inter-partition join. However, the minimum property-cut partitioning is a NP-hard problem and we propose a heuristic greedy algorithm to address that. Extensive experiments over a variety of synthetic and real RDF graphs show that the proposed technique can significantly avoid joins and results in good performance.

Estimating the accuracy of an automatically constructed knowledge graph (KG) becomes a challenging task as the KG often contains a large number of entities and triples. Generally, two major components information extraction (IE) and entity linking (EL) are involved in KG construction. However, the existing approaches just focus on evaluating the triple accuracy that indicates the IE quality, completely ignoring the entity accuracy. Motivated by the fact that the major advance of machines is the strong computing power while humans are skilled in correctness verification, we propose an efficient interactive method to reduce the overall cost for evaluating the KG quality, which produces accuracy estimates with a statistical guarantee for both triples and entities. Instead of annotating triples and entities separately, we design a general annotation cost that blends triples and entities generated from the identical source text. During human verification, the machine can pre-compute and infer triples to be annotated in the next round by speculating human feedback. The human-machine collaborative mechanism is optimized by formulating an order selection problem of triples which is NP-hard. Thus, a Monte Carlo Tree Search is proposed to guide the annotation process by finding an approximate solution. Extensive experiments demonstrate that our method takes less annotation cost while yielding higher accuracy estimation quality compared to the state-of-the-art approaches.

The growing popularity of dynamic applications such as social networks provides a promising way to detect valuable information in real time. These applications create high-speed data that can be easily modeled as streaming graph. Efficient analysis over these data is of great significance. In this paper, we study the subgraph (isomorphism) search over streaming graph data that obeys timing order constraints over the occurrence of edges in the stream. The sliding window model is employed to focus on the most recent data. We propose an efficient solution to answer subgraph search, introduce optimizations to greatly reduce the space cost, and design concurrency management to improve system throughput. Extensive experiments on real network traffic data and synthetic social streaming data shows that our solution outperforms comparative ones by one order of magnitude with less space cost.

Real-world graphs exhibit many interesting properties that differentiate them from random graphs, which have been extensively studied for the past decades. For various proposed generative models, a majority of them build the graph by sequentially adding each node and the attached edges. However, the growth of many real-world graphs, such as social networks, is naturally modeled by the sequential insertion of edges. Unfortunately, to the best of our knowledge, no generative model has been proposed to reveal this process. We propose the first sequence-of-edges model, denoted astemporal preferential attachment (TPA). It relies onpreferential attachment (PA), one of the most influential mechanisms to generate scale-free graphs, and takes time-decay effect and node fitness into consideration. Empirical analysis demonstrates that our model preserves several key properties of the real-world graphs, including both the properties observed from the snapshot graphs (e.g., power-law distribution) and temporal properties observed from the graph generation process (e.g., shrinking diameter). Meanwhile, our model is sufficiently general to accommodate several forms of time decay and fitness distributions. Then, we design two efficient algorithms that generate TPA graphs with billions of edges in several minutes.

Graph Edit Distance (GED) is a classical graph similarity metric that can be tailored to a wide range of applications. However, the exact GED computation is NP-complete, which means it is only feasible for small graphs only. And therefore, approximate GED computation methods are used in most real-world applications. However, traditional practices and end-to-end learning-based methods have their shortcomings when applied for approximate GED computation. The former relies on experience and usually performs not well. The latter is only capable of computing similarity scores between graphs without an actual edit path, which is crucial in specific problems (e.g., Graph Alignment, Semantic Role Labeling). This paper proposes a novel approach Noah, which combines A* search algorithm and graph neural networks to compute approximate GED in a more effective and intelligent way. The combination is mainly reflected in two aspects. First, we learn the estimated cost function h(·) by Graph Path Networks. Pre-training GEDs and corresponding edit paths are also incorporated for training the model, therefore helping optimize the search direction of A* search algorithm. Second, we learn an elastic beam size that can help reduce search size and satisfy various user settings. Experimental results demonstrate the practical effectiveness of our approach on several tasks and suggest that our approach significantly outperforms the state-of-the-art methods.

Streaming graph analysis is gaining importance in various fields due to the natural dynamicity in many real graph applications. However, approximately counting triangles in real-world streaming graphs with edge duplication and expiration remains an unsolved problem. In this paper, we propose SWTC algorithm to address approximate sliding-window triangle counting problem in streaming graphs with edge duplication. In SWTC, we propose a fixed-length slicing strategy that addresses both unbiased sampling and cardinality estimation issues with a bounded memory usage. We theoretically prove the superiority of our method in sample graph size and estimation accuracy under given memory upper bound. Extensive experiments also confirm that our approach has higher accuracy compared with the baseline method under the same memory usage.

Triangle counting is an important problem in graph mining, which has achieved great performance improvement on GPU in recent years. Instead of proposing a new GPU triangle counting algorithm, in this paper, we propose a novel lightweight graph preprocessing method to boost many state-of-the-art GPU triangle counting algorithms without changing their implementations and data structures. Specifically, we find common computing patterns in existing algorithms, and abstract two analytic models to measure how workload imbalance and diversity in these computing patterns affect performance exactly. Then, due to the NP-hardness of the model optimization, we propose approximate solutions by determining edge directions to balance workloads and reordering vertices to maximize the degree of parallelism within GPU blocks. Finally, extensive experiments confirm the significant performance improvement and high usability of our approach.

Graph similarity search under the graph edit distance constraint has received considerable attention in many applications, such as bioinformatics, data mining, pattern recognition and social networks. Existing methods for this problem have limited scalability because of the huge amount of memory they consume when handling very large graph databases with tens of millions of graphs. In this article, we present a succinct index that incorporates succinct data structures and hybrid encoding to achieve improved query time performance with minimal space usage. Specifically, the space usage of our index requires only 5-15 percent of the previous state-of-the-art indexing size while at the same time achieving several times acceleration in query time on the tested data. We also improve the query performance by augmenting the global filter with range searching, which allows us to perform similarity search in a reduced region. In addition, we propose two effective lower bounds together with a boosting technique to obtain the smallest possible candidate set. Extensive experiments demonstrate that our proposed approach is superior both in space and filtering to the state-of-the-art approaches. To the best of our knowledge, our index is the first in-memory index for this problem that successfully scales to cope with the large dataset of 25 million chemical structure graphs from the PubChem dataset. The source code is available online.

This paper revisits the classical problem of multiple query optimization in federated RDF systems. We propose a heuristic query rewriting-based approach to optimize the evaluation of multiple queries. This approach can take advantage of SPARQL 1.1 to share the common computation of multiple queries while considering the cost of both query evaluation and data shipment. Although we prove that finding the optimal rewriting for multiple queries is NP-complete, we propose a heuristic rewriting algorithm with a bounded approximation ratio. Furthermore, we propose an efficient method to use the interconnection topology between RDF sources to filter out irrelevant sources, and utilize some characteristics of SPARQL 1.1 to optimize multiple joins of intermediate matches. The extensive experimental studies show that the proposed techniques are effective, efficient and scalable.

Subgraph isomorphism is a well-known NP-hard problem that is widely used in many applications, such as social network analysis and querying over the knowledge graph. Due to the inherent hardness, its performance is often a bottleneck in various real-world applications. We address this by designing an efficient subgraph isomorphism algorithm leveraging features of GPU architecture, such as massive parallelism and memory hierarchy. Existing GPU-based solutions adopt two-step output scheme, performing the same join twice in order to write inter-mediate results concurrently. They also lack GPU architecture-aware optimizations that allow scaling to large graphs. In this paper, we propose a GPU-friendly subgraph isomorphism algorithm, GSI. Different from existing edge join-based GPU solutions, we propose a Prealloc-Combine strategy based on the vertex-oriented framework, which avoids joining-twice in existing solutions. Also, a GPU-friendly data structure (called PCSR) is proposed to represent an edge-labeled graph. Extensive experiments on both synthetic and real graphs show that GSI outperforms the state-of-the-art algorithms by up to several orders of magnitude and has good scalability with graph size scaling to hundreds of millions of edges.

Large scale knowledge graph (KG) has attracted wide attentions in both academia and industry recently. However, due to the complexity of SPARQL syntax and massive volume of real KG, it remains difficult for ordinary users to access KG. In this demo, we present VISION-KG, a topic-centric visualization system to help users navigate KG easily via entity summarization and entity clustering. Given a query entity v0, VISION-KG summarizes the induced subgraph of v0's neighbor nodes via our proposed facts ranking method that measures importance, relatedness and diversity. Moreover, to achieve conciseness, we split the summarized graph into several topic-centric summarized subgraph according to semantic and structural similarities among entities. We will demonstrate how VISION-KG provides a user-friendly visualization interface for navigating KG.

Semantic parsing-based RDF question answering (QA) systems are to interpret users' natural language questions as query graphs and return answers over RDF repository. However, due to the complexity of linking natural phrases with specific RDF items (e.g., entities and predicates), it remains difficult to understand users' question sentences precisely, hence QA systems may not meet users' expectation, offering wrong answers and dismissing some correct answers. In this paper, we design an I nteractive M echanism aiming for PRO motion V ia users' fe edback to Q A systems (IMPROVE-QA), a whole framework to not only make existing QA systems return more precise answers based on a few feedbacks over the original answers given by RDF QA systems, but also enhance paraphrasing dictionaries to ensure a continuous-learning capability in improving RDF QA systems. To provide better interactivity and online performance, we design a holistic graph mining algorithm (HWspan) to automatically refine the query graph. Extensive experiments on both Freebase and DBpedia confirm the effectiveness and superiority of our approach.

Partial evaluation has recently been used for processing SPARQL queries over a large resource description framework (RDF) graph in a distributed environment. However, the previous approach is inefficient when dealing with complex queries. In this study, we further improve the "partial evaluation and assembly" framework for answering SPARQL queries over a distributed RDF graph, while providing performance guarantees. Our key idea is to explore the intrinsic structural characteristics of partial matches to filter out irrelevant partial results while providing performance guarantees on the data shipment and the response time. We also propose an efficient assembly algorithm to utilize the characteristics of partial matches to merge them and form final results. To improve the efficiency of finding partial matches further, we propose an optimization that communicates variables' candidates among sites to avoid redundant computations. In addition, although our approach is partitioning-tolerant, different partitioning strategies result in different performances, and we evaluate different partitioning strategies for our approach. Experiments over both real and synthetic RDF datasets confirm the superiority of our approach.

A graph stream is a continuous sequence of data items, in which each item indicates an edge, including its two endpoints and edge weight. It forms a dynamic graph that changes with every item. Graph streams play important roles in cyber security, social networks, cloud troubleshooting systems and more. Due to the vast volume and high update speed of graph streams, traditional data structures for graph storage such as the adjacency matrix and the adjacency list are no longer sufficient. However, prior art of graph stream summarization, like CM sketches, gSketches, TCM and gMatrix, either supports limited kinds of queries or suffers from poor accuracy of query results. In this paper, we propose a novel Graph Stream Sketch (GSS for short) to summarize the graph streams, which has linear space cost O(|E|) (E is the edge set of the graph) and constant update time cost (O(1)) and supports most kinds of queries over graph streams with the controllable errors. Both theoretical analysis and experiment results confirm the superiority of our solution with regard to the time/space complexity and query results' precision compared with the state-of-the-art.

The growing popularity of dynamic applications such as social networks provides a promising way to detect valuable information in real time. These applications create high-speed data that can be easily modeled as streaming graph. Efficient analysis over these data is of great significance. In this paper, we study the subgraph (isomorphism) search over streaming graph data that obeys timing order constraints over the occurrence of edges in the stream. We propose a solution to efficiently answer subgraph search, introduce optimizations to greatly reduce the space cost, and design concurrency management to improve system throughput. Extensive experiments on real network traffic data and synthetic social streaming data confirms the efficiency and effectiveness of our solution.

As massive volumes of Resource Description Framework (RDF) data are growing, designing a distributed RDF database system to manage them is necessary. In designing this system, it is very common to partition the RDF data into some parts, called fragments, which are then distributed. Thus, the distribution design comprises two steps: fragmentation and allocation. In this study, we explore the workload for fragmentation and allocation, which aims to reduce the communication cost during SPARQL query processing. Specifically, we adaptively maintain some frequent access patterns (FAPs) to reflect the characteristics of the workload while ensuring the data integrity and approximation ratio. Based on these frequent access patterns, we propose three fragmentation strategies, namely vertical, horizontal, and mixed fragmentation, to divide RDF graphs while meeting different types of query processing objectives. After fragmentation, we discuss how to allocate these fragments to various sites while balancing the fragments. Finally, we discuss how to process queries based on the results of fragmentation and allocation. Experiments over large RDF datasets confirm the superior performance of our proposed solutions.

RDF question/answering (Q/A) allows users to ask questions in natural languages over a knowledge base represented by RDF. To answer a natural language question, the existing works focus on question understanding to deal with the disambiguation of phrases linking, which ignore the query composition and execution. In this paper, we propose a systematic framework to answer natural language questions over RDF repository (RDF Q/A) from a graph data-driven perspective. We propose the (super) semantic query graph to model the query intention in the natural language question in a structural way, based on which, RDF Q/A is reduced to subgraph matching problem. More importantly, we resolve the ambiguity both of phrases and structures at the time when matches of query are found. To build the super semantic query graph, we propose a node-first framework which has high robustness and can tackle with complex questions. Extensive experiments confirm that our method not only improves the precision but also speeds up query performance greatly.

With the prevalent trend of combining online and offline interactions among users in event-based social networks (EBSNs), event recommendation has become an essential means to help people discover new interesting events to attend. However, existing literatures on event recommendations ignore the social attribute of events: people prefer to attend events with their friends or family rather than alone. Therefore, we propose a new recommendation paradigm: joint event-partner recommendation that focuses on recommending event-partner pairs to users. In this paper, we focus on the new problem of joint event-partner recommendation in EBSNs, which is extremely challenging due to the intrinsic cold-start property of events, the complex decision-making process for choosing event-partner pairs and the huge prediction space of event-partner combinations. We propose a generic graph-based embedding model (GEM) to collectively embed all the observed relations among users, events, locations, time and text content in a shared low-dimension space, which is able to leverage the correlation between events and their associated content and contextual information to address the cold-start issue effectively. To accelerate the convergence of GEM and improve its modeling accuracy, an adaptive noise sampler is developed to generate adversarial negative samples in the model optimization. Besides, to speed up the online recommendation, we propose a novel space transformation method to project each event-partner pair to one point in a new space and then develop effective space pruning and efficient online recommendation techniques. We conduct comprehensive experiments on our created real benchmark datasets, and the experimental results demonstrate the superiority of our proposals in terms of recommendation effectiveness, efficiency and scalability

Most existing algorithms computing the maximum independent set (MIS) or independent set (IS) are designed for handling static graphs, which may not be practicable as many networks are dynamically evolving over time. In this paper, we study the MIS/IS problem in evolving graphs by considering graph update operations: vertex/edge addition and vertex/edge deletion. Instead of computing the MIS/IS of the updated graph from scratch, we propose a baseline algorithm that finds the MIS/IS at time t i+1 based on the MIS/IS at time ti. Due to the hardness of computing an exact MIS, we develop an efficient constant-time algorithm LSTwo to return a high-quality (large-size) independent set. Then we design a lazy search algorithm which produces higher-quality independent sets. To improve the time efficiency further, we devise the conditional besieging and k-petal based methods to reduce the search space. Extensive experimental studies over large-scale graphs confirm the effectiveness and efficiency of our proposed algorithms.

Data stream processing is a fundamental issue in many fields, such as data mining, databases, network traffic measurement. There are five typical tasks in data stream processing: frequency estimation, heavy hitter detection, heavy change detection, frequency distribution estimation, and entropy estimation. Different algorithms are proposed for different tasks, but they seldom achieve high accuracy and high speed at the same time. To address this issue, we propose a novel data structure named HeavyGuardian. The key idea is to intelligently separate and guard the information of hot items while approximately record the frequencies of cold items. We deploy HeavyGuardian on the above five typical tasks. Extensive experimental results show that HeavyGuardian achieves both much higher accuracy and higher speed than the state-of-the-art solutions for each of the five typical tasks. The source codes of HeavyGuardian and other related algorithms are available at GitHub.

In this paper, we focus on accelerating a widely employed computing pattern --- set intersection, to boost a group of graph algorithms. Graph's adjacency-lists can be naturally considered as node sets, thus set intersection is a primitive operation in many graph algorithms. We propose QFilter, a set intersection algorithm using SIMD instructions. QFilter adopts a merge-based framework and compares two blocks of elements iteratively by SIMD instructions. The key insight for our improvement is that we quickly filter out most of unnecessary comparisons in one byte-checking step. We also present a binary representation called BSR that encodes sets in a compact layout. By combining QFilter and BSR, we achieve data-parallelism in two levels --- inter-chunk and intra-chunk parallelism. Moreover, we find that node ordering impacts the performance of intersection by affecting the compactness of BSR. We formulate the graph reordering problem as an optimization of the compactness of BSR, and prove its strong NP-completeness. Thus we propose an approximate algorithm that can find a better ordering to enhance the intra-chunk parallelism. We conduct extensive experiments to confirm that our approach can improve the performance of set intersection in graph algorithms significantly.

RDF Question/Answering(Q/A) systems can interpret user's question N as SPARQL query Q and return answer set $Q(D)$ over RDF repository D to the user. However, due to the complexity of linking natural phrases with specific RDF items (e.g., entities and predicates), it remains difficult to understand users' questions precisely, hence $Q(D)$ may not meet users' expectation, offering wrong answers and dismissing some correct answers. In this demo, we design an I Interactive Mechanism aiming for PRO motion V ia feedback to Q/A systems (IMPROVE-QA), a whole platform to make existing Q/A systems return more precise answers (denoted as $\mathcal Q^\prime (D)$) to users. Based on user's feedback over $Q(D)$, IMPROVE-QA automatically refines the original query Q into a new query graph $\mathcal Q^\prime $ with minimum modifications, where $\mathcal Q^\prime (D)$ provides more precise answers. We will also demonstrate how IMPROVE-QA can apply the "lesson'' learned from the user in each query to improve the precision of Q/A systems on subsequent natural language questions.

The gap between unstructured natural language and structured data makes it challenging to build a system that supports using natural language to query large knowledge graphs. Many existing methods construct a structured query for the input question based on a syntactic parser. Once the input question is parsed incorrectly, a false structured query will be generated, which may result in false or incomplete answers. The problem gets worse especially for complex questions. In this paper, we propose a novel systematic method to understand natural language questions by using a large number of binary templates rather than semantic parsers. As sufficient templates are critical in the procedure, we present a low-cost approach that can build a huge number of templates automatically. To reduce the search space, we carefully devise an index to facilitate the online template decomposition. Moreover, we design effective strategies to perform the two-level disambiguations (i.e., entity-level ambiguity and structure-level ambiguity) by considering the query semantics. Extensive experiments over several benchmarks demonstrate that our proposed approach is effective as it significantly outperforms state-of-the-art methods in terms of both precision and recall.

Identifying events from different sources is essential to various business process applications such as provenance querying or process mining. Distinct features of heterogeneous events, including opaque names and dislocated traces, prevent existing data integration techniques from performing well. To address these issues, in this paper, (1) we propose an event similarity function by iteratively evaluating similar neighbors. (2) In addition to event nodes, we further employ the similarity of edges (indicating relationships among events) in event matching. We prove NP-hardness of finding the optimal event matching w.r.t. node and edge similarities, and propose an efficient heuristic for event matching. Experiments demonstrate that the proposed event matching approach can achieve significantly higher accuracy than state-of-the-art matching methods. In particular, by considering the event edge similarity, our heuristic matching algorithm further improves the matching accuracy without introducing much overhead.

RDF question/answering (Q/A) allows users to ask questions in natural languages over a knowledge base represented by RDF. To answer a natural language question, the existing work takes a two-stage approach: question understanding and query evaluation. Their focus is on question understanding to deal with the disambiguation of the natural language phrases. The most common technique is the joint disambiguation, which has the exponential search space. In this paper, we propose a systematic framework to answer natural language questions over RDF repository (RDF Q/A) from a graph data-driven perspective. We propose a semantic query graph to model the query intention in the natural language question in a structural way, based on which, RDF Q/A is reduced to subgraph matching problem. More importantly, we resolve the ambiguity of natural language questions at the time when matches of query are found. The cost of disambiguation is saved if there are no matching found. More specifically, we propose two different frameworks to build the semantic query graph, one is relation (edge)-first and the other one is node-first. We compare our method with some state-of-the-art RDF Q/A systems in the benchmark dataset. Extensive experiments confirm that our method not only improves the precision but also speeds up query performance greatly.

In this paper, we propose a data structure, a quadruple neighbor list (QN-list, for short), to support real time queries of all longest increasing subsequence (LIS) and LIS with constraints over sequential data streams. The QN-List built by our algorithm requires O(w) space, where w is the time window size. The running time for building the initial QN-List takes O(w logw) time. Applying the QN-List, insertion of the new item takes O(logw) time and deletion of the first item takes O(w) time. To the best of our knowledge, this is the first work to support both LIS enumeration and LIS with constraints computation by using a single uniform data structure for real time sequential data streams. Our method outperforms the state-of-the-art methods in both time and space cost, not only theoretically, but also empirically.

Keyword search provides ordinary users an easy-to-use interface for querying RDF data. Given the input keywords, in this paper, we study how to assemble a query graph that is to represent user's query intention accurately and efficiently. Based on the input keywords, we first obtain the elementary query graph building blocks, such as entity/class vertices and predicate edges. Then, we formally define the query graph assembly (QGA) problem. Unfortunately, we prove theoretically that QGA is a NP-complete problem. In order to solve that, we design some heuristic lower bounds and propose a bipartite graph matching-based best-first search algorithm. The algorithm's time complexity is O(k2l ... l3l), where l is the number of the keywords and k is a tunable parameter, i.e., the maximum number of candidate entity/class vertices and predicate edges allowed to match each keyword. Although QGA is intractable, both l and k are small in practice. Furthermore, the algorithm's time complexity does not depend on the RDF graph size, which guarantees the good scalability of our system in large RDF graphs. Experiments on DBpedia and Freebase confirm the superiority of our system on both effectiveness and efficiency.

The ever-increasing knowledge graphs impose an urgent demand of providing effective and easy-to-use query techniques for end users. Structured query languages, such as SPARQL, offer a powerful expression ability to query RDF datasets. However, they are difficult to use. Keywords are simple but have a very limited expression ability. Natural language question (NLQ) is promising on querying knowledge graphs. A huge challenge is how to understand the question clearly so as to translate the unstructured question into a structured query. In this paper, we present a data + oracle approach to answer NLQs over knowledge graphs. We let users verify the ambiguities during the query understanding. To reduce the interaction cost, we formalize an interaction problem and design an efficient strategy to solve the problem. We also propose a query prefetch technique by exploiting the latency in the interactions with users. Extensive experiments over the QALD dataset demonstrate that our proposed approach is effective as it outperforms state-of-the-art methods in terms of both precision and recall.

Extracting keyphrases from documents automatically is an important and interesting task since keyphrases provide a quick summarization for documents. Although lots of efforts have been made on keyphrase extraction, most of the existing methods (the co-occurrence-based methods and the statistic-based methods) do not take semantics into full consideration. The co-occurrence-based methods heavily depend on the co-occurrence relations between two words in the input document, which may ignore many semantic relations. The statistic-based methods exploit the external text corpus to enrich the document, which introduce more unrelated relations inevitably. In this paper, we propose a novel approach to extract keyphrases using knowledge graphs, based on which we could detect the latent relations of two keyterms (i.e., noun words and named entities) without introducing many noises. Extensive experiments over real data show that our method outperforms the state-of-the-art methods including the graph-based co-occurrence methods and statistic-based clustering methods.

The increasing size of RDF data requires efficient systems to store and query them. There have been efforts to map RDF data to a relational representation, and a number of systems exist that follow this approach. We have been investigating an alternative approach of maintaining the native graph model to represent RDF data, and utilizing graph database techniques (such as a structure-aware index and a graph matching algorithm) to address RDF data management. Since 2009, we have been developing a set of graph-based RDF data management systems that follow this approach: gStore, gStore-D and gAnswer. The first two are designed to support efficient SPARQL query evaluation in a centralized and distributed/parallel environments, respectively, while the last one aims to provide an easy-to-use interface (natural language question/answering) for users to access a RDF repository. In this paper, we give an overview of these systems and also discuss our design philosophy.

Graphs have been widely used to model complex data in many real-world applications. Answering vertex join queries over large graphs is meaningful and interesting, which can benefit friend recommendation in social networks and link prediction, and so on. In this article, we adopt “SimRank” [13] to evaluate the similarity between two vertices in a large graph because of its generality. Note that “Simank” is purely structure dependent, and it does not rely on the domain knowledge. Specifically, we define a S im R ank-based j oin ( SRJ ) query to find all vertex pairs satisfying the threshold from two sets of vertices U and V . To reduce the search space, we propose a shortest-path-distance-based upper bound for SimRank scores to prune unpromising vertex pairs. In the verification, we propose a novel index, called h-go cover + , to efficiently compute the SimRank score of any single vertex pair. Given a graph G , we only materialize the SimRank scores of a small proportion of vertex pairs (i.e., the h-go cover + vertex pairs), based on which the SimRank score of any vertex pair can be computed easily. To find the h-go cover + vertex pairs, we propose an efficient method without building the vertex-pair graph. Hence, large graphs can be dealt with easily. Extensive experiments over both real and synthetic datasets confirm the efficiency of our solution.

As the volume of the RDF data becomes increasingly large, it is essential for us to design a distributed database system to manage it. For distributed RDF data design, it is quite common to partition the RDF data into some parts, called fragments, which are then distributed. Thus, the distribution design consists of two steps: fragmentation and allocation. In this paper, we propose a method to explore the intrinsic similarities among the structures of queries in a workload for fragmentation and allocation, which aims to reduce the number of crossing matches and the communication cost during SPARQL query processing. Specifically, we mine and select some frequent access patterns to reflect the characteristics of the workload. Based on the selected frequent access patterns, we propose two fragmentation strategies, vertical and horizontal fragmentation strategies, to divide RDF graphs while meeting different kinds of query processing objectives. Vertical fragmentation is for better throughput and horizontal fragmentation is for better performance. After fragmentation, we discuss how to allocate these fragments to various sites. Finally, we discuss how to process a query based on the results of fragmentation and allocation. Extensive experiments confirm the superior performance of our proposed solutions.

The wide presence of large graph data and the increasing popularity of storing data in the cloud drive the needs for graph query processing on a remote cloud. But a fundamental challenge is to process user queries without compromising sensitive information. This work focuses on privacy preserving subgraph matching in a cloud server. The goal is to minimize the overhead on both cloud and client sides for subgraph matching, without compromising users' sensitive information. To that end, we transform an original graph $G$ into a privacy preserving graph Gk, which meets the requirement of an existing privacy model known as k-automorphism. By making use of the symmetry in a k-automorphic graph, a subgraph matching query can be efficiently answered using a graph Go, a small subset of Gk. This approach saves both space and query cost in the cloud server. We also anonymize the query graphs to protect their label information using label generalization technique. To reduce the search space for a subgraph matching query, we propose a cost model to select the more effective label combinations. The effectiveness and efficiency of our method are demonstrated through extensive experimental results on real datasets.

In this paper, we propose a data structure, a quadruple neighbor list (QN-list, for short), to support real time queries of all longest increasing subsequence (LIS) and LIS with constraints over sequential data streams. The QN-List built by our algorithm requires O ( w ) space, where w is the time window size. The running time for building the initial QN-List takes O ( w log w ) time. Applying the QN-List, insertion of the new item takes O (log w ) time and deletion of the first item takes O ( w ) time. To the best of our knowledge, this is the first work to support both LIS enumeration and LIS with constraints computation by using a single uniform data structure for real time sequential data streams. Our method outperforms the state-of-the-art methods in both time and space cost, not only theoretically, but also empirically.

RDF knowledge graphs have attracted increasing attentions these years. However, due to the schema-free nature of RDF data, it is very difficult for users to have full knowledge of the underlying schema. Furthermore, the same kind of information can be represented in diverse graph fragments. Hence, it is a huge challenge to formulate complex SPARQL expressions by taking the union of all possible structures. In this paper, we propose an effective framework to access the RDF repository even if users have no full knowledge of the underlying schema. Specifically, given a SPARQL query, the system could return as more answers that match the query based on the semantic similarity as possible. Interestingly, we propose a systematic method to mine diverse semantically equivalent structure patterns. More importantly, incorporating both structural and semantic similarities we are the first to propose a novel similarity measure, semantic graph edit distance . In order to improve the efficiency performance, we apply the semantic summary graph to summarize the knowledge graph, which supports both high-level pruning and drill-down pruning. We also devise an effective lower bound based on the TA-style access to each of the candidate sets. Extensive experiments over real datasets confirm the effectiveness and efficiency of our approach.

Subgraph search is very useful in many real-world applications. However, users may be overwhelmed by the masses of matches. In this paper, we propose a subgraph skyline analysis problem, denoted as S2A, to support more complicated analysis over graph data. Specifically, given a large graph G and a query graph q, we want to find all the subgraphs g in G, such that g is graph isomorphic to q and not dominated by any other subgraphs. In order to improve the efficiency, we devise a hybrid feature encoding incorporating both structural and numeric features based on a partitioning strategy, and discuss how to optimize the space partitioning. We also present a skylayer index to facilitate the dynamic subgraph skyline computation. Moreover, an attribute cluster-based method is proposed to deal with the curse of dimensionality. Extensive experiments over real datasets confirm the effectiveness and efficiency of our algorithm.

Many types of human mobility data, such as flows of taxicabs, card swiping data of subways, bike trip data and Call Details Records (CDR), can be modeled by a Spatio-Temporal Graph (STG). STG is a directed graph in which vertices and edges are associated with spatio-temporal properties (e.g. the traffic flow on a road and the geospatial location of an intersection). In this paper, we instantly detect interesting phenomena, entitled black holes and volcanos, from an STG. Specifically, a black hole is a subgraph (of an STG) that has the overall inflow greater than the overall outflow by a threshold, while a volcano is a subgraph with the overall outflow greater than the overall inflow by a threshold (detecting volcanos from an STG is proved to be equivalent to the detection of black holes). The online detection of black holes/volcanos can timely reflect anomalous events, such as disasters, catastrophic accidents, and therefore help keep public safety. The patterns of black holes/volcanos and the relations between them reveal human mobility patterns in a city, thus help formulate a better city planning or improve a system's operation efficiency. Based on a well-designed STG index, we propose a two-step black hole detection algorithm: The first step identifies a set of candidate grid cells to start from; the second step expands an initial edge in a candidate cell to a black hole and prunes other candidate cells after a black hole is detected. Then, we adapt this detection algorithm to a continuous black hole detection scenario. We evaluate our method based on Beijing taxicab data and the bike trip data in New York, finding urban anomalies and human mobility patterns.

In this demonstration, we present the gStore RDF triple store. gStore is based on graph encoding and subgraph match, distinct from many other systems. More importantly, it can handle, in a uniform manner, different data types (strings and numerical data) and SPARQL queries with wildcards, aggregate, range and top-k operators over dynamic RDF datasets. We will demonstrate the main features of our system, show how to search Wikipedia documents using gStore and how to build users' own application using gStore through C++/Java API.

A challenging task in the natural language question answering (Q/A for short) over RDF knowledge graph is how to bridge the gap between unstructured natural language questions (NLQ) and graph-structured RDF data (GOne of the effective tools is the "template", which is often used in many existing RDF Q/A systems. However, few of them study how to generate templates automatically. To the best of our knowledge, we are the first to propose a join approach for template generation. Given a workload D of SPARQL queries and a set N of natural language questions, the goal is to find some pairs q, n, for q∈ D ∧ n ∈, N, where SPARQL query q is the best match for natural language question n. These pairs provide promising hints for automatic template generation. Due to the ambiguity of the natural languages, we model the problem above as an uncertain graph join task. We propose several structural and probability pruning techniques to speed up joining. Extensive experiments over real RDF Q/A benchmark datasets confirm both the effectiveness and efficiency of our approach.

Recommender systems have been studied comprehensively in both academic and industrial fields over the past decade. As user interests can be affected by context at any time and any place in mobile scenarios, rich context information becomes more and more important for personalized context-aware recommendations. Although existing context-aware recommender systems can make context-aware recommendations to some extent, they suffer several inherent weaknesses: (1) Users’ context-aware interests are not modeled realistically, which reduces the recommendation quality; (2) Current context-aware recommender systems ignore trust relations among users. Trust relations are actually context-aware and associated with certain aspects (i.e., categories of items) in mobile scenarios. In this article, we define a term role to model common context-aware interests among a group of users. We propose an efficient role mining algorithm to mine roles from a “user-context-behavior” matrix, and a role-based trust model to calculate context-aware trust value between two users. During online recommendation, given a user u in a context c , an efficient weighted set similarity query (WSSQ) algorithm is designed to build u ’s role-based trust network in context c . Finally, we make recommendations to u based on u ’s role-based trust network by considering both context-aware roles and trust relations. Extensive experiments demonstrate that our recommendation approach outperforms the state-of-the-art methods in both effectiveness and efficiency.

In real-world graphs such as social networks, Semantic Web and biological networks, each vertex usually contains rich information, which can be modeled by a set of tokens or elements. In this paper, we study a subgraph matching with set similarity (SMS2) query over a large graph database, which retrieves subgraphs that are structurally isomorphic to the query graph, and meanwhile satisfy the condition of vertex pair matching with the (dynamic) weighted set similarity. To efficiently process the SMS2query, this paper designs a novel lattice-based index for data graph, and lightweight signatures for both query vertices and data vertices. Based on the index and signatures, we propose an efficient two-phase pruning strategy including set similarity pruning and structure-based pruning, which exploits the unique features of both (dynamic) weighted set similarity and graph topology. We also propose an efficient dominating-set-based subgraph matching algorithm guided by a dominating set selection algorithm to achieve better query performance. Extensive experiments on both real and synthetic datasets demonstrate that our method outperforms state-of-the-art methods by an order of magnitude.

Since many graph data are often noisy and incomplete in real applications, it has become increasingly important to retrieve graphs g in the graph database D that approximately match the query graph q, rather than exact graph matching. In this paper, we study the problem of graph similarity search, which retrieves graphs that are similar to a given query graph under the constraint of graph edit distance. We propose a systematic method for edit-distance based similarity search problem. Specifically, we derive two lower bounds, i.e., partition-based and branch-based bounds, from different perspectives. More importantly, a hybrid lower bound incorporating both ideas of the two lower bounds is proposed, which is theoretically proved to have higher (at least not lower) pruning power than using the two lower bounds together. We also present a uniform index structure, namely u-tree, to facilitate effective pruning and efficient query processing. Extensive experiments confirm that our proposed approach outperforms the existing approaches significantly, in terms of both the pruning power and query response time.

Subgraph search is very useful in many real-world applications. However, users may be overwhelmed by the masses of matches. In this paper, we propose subgraph skyline search problem, denoted as S3, to support more complicated analysis over graph data. Specifically, given a large graph G and a query graph q, we want to find all the subgraphs g in G, such that g is graph isomorphic to q and not dominated by any other subgraphs. In order to improve the efficiency, we devise a hybrid feature encoding incorporating both structural and numeric features. Moreover, we present some optimizations based on partitioning strategy. We also propose a skylayer index to facilitate the dynamic subgraph skyline computation. Extensive experiments over real dataset confirm the effectiveness and efficiency of our algorithm.

In this paper, we present a spatiotemporal information integrated RDF data management system, called gst-Store. In gst-Store, some entities have spatiotemporal features, and some statements have valid time intervals and occurring locations. We introduce some spatiotemporal assertions into the SPARQL query language to an-swer the spatiotemporal range queries and join queries. Some ex-amples are listed to demonstrate our demo.

Identifying duplicate events are essential to various business process applications such as provenance querying or process mining. Distinct features of heterogeneous events including opaque names, dislocated traces and composite events, prevent existing data integration from techniques performing well. To address these issues, in this paper, we propose an event similarity function by iteratively evaluating similar neighbors. We prove the convergence of iterative similarity computation, and propose several pruning and estimation methods. To efficiently support matching composite events, we devise upper bounds of event similarities. Experiments on real and synthetic datasets demonstrate that the proposed event matching approaches can achieve significantly higher accuracy than the state-of-the-art matching methods.

RDF question/answering (Q/A) allows users to ask questions in natural languages over a knowledge base represented by RDF. To answer a national language question, the existing work takes a two-stage approach: question understanding and query evaluation. Their focus is on question understanding to deal with the disambiguation of the natural language phrases. The most common technique is the joint disambiguation, which has the exponential search space. In this paper, we propose a systematic framework to answer natural language questions over RDF repository (RDF Q/A) from a graph data-driven perspective. We propose a semantic query graph to model the query intention in the natural language question in a structural way, based on which, RDF Q/A is reduced to subgraph matching problem. More importantly, we resolve the ambiguity of natural language questions at the time when matches of query are found. The cost of disambiguation is saved if there are no matching found. We compare our method with some state-of-the-art RDF Q/A systems in the benchmark dataset. Extensive experiments confirm that our method not only improves the precision but also speeds up query performance greatly.

Due to many real applications of graph databases, it has become increasingly important to retrieve graphs g (in graph database D) that approximately match with query graph q, rather than exact subgraph matches. In this paper, we study the problem of graph similarity search, which retrieves graphs that are similar to a given query graph under the constraint of the minimum edit distance. Specifically, we derive a lower bound, branch-based bound, which can greatly reduce the search space of the graph similarity search. We also propose a tree index structure, namely b-tree, to facilitate effective pruning and efficient query processing. Extensive experiments confirm that our proposed approach outperforms the existing approaches by orders of magnitude, in terms of both pruning power and query response time.

As more and more RDF data becomes available, such as DBpedia, Yago and Freebase, it is desired to provide users with simple interfaces to access the datasets. Although the SPARQL query language is a standard way to query RDF data, it remains tedious and difficult even for expert users because of the formality of the language and the complexity of the underlying schema of RDF data. An ideal system should allow users to express queries in their own languages. In this work, we propose a methodology to translate natural language questions into SPARQL queries, which can be answered by existing RDF engines and fulfill users? information need.

Graphs have been widely used to model complex data in many real-world applications. Answering vertex join queries over large graphs is meaningful and interesting, which can benefit friend recommendation in social networks and link prediction, etc. In this paper, we adopt "SimRank" to evaluate the similarity of two vertices in a large graph because of its generality. Note that "SimRank" is purely structure dependent and it does not rely on the domain knowledge. Specifically, we define a SimRank-based join (SRJ) query to find all the vertex pairs satisfying the threshold in a data graphG. In order to reduce the search space, we propose an estimated shortest-path distance based upper bound for SimRank scores to prune unpromising vertex pairs. In the verification, we propose a novel index, called h-go cover, to efficiently compute the SimRank score of a single vertex pair. Given a graphG, we only materialize the SimRank scores of a small proportion of vertex pairs (called h-go covers), based on which, the SimRank score of any vertex pair can be computed easily. In order to handle large graphs, we extend our technique to the partition-based framework. Thorough theoretical analysis and extensive experiments over both real and synthetic datasets confirm the efficiency and effectiveness of our solution.

In this paper, we study a variant of reachability queries, called label-constraint reachability (LCR) queries, specifically,given a label set S and two vertices u1 and u2 in a large directed graph G, we verify whether there exists a path from u1 to u2 under label constraint S. Like traditional reachability queries, LCR queries are very useful, such as pathway finding in biological networks, inferring over RDF (resource description f ramework) graphs, relationship finding in social networks. However, LCR queries are much more complicated than their traditional counterpart.Several techniques are proposed in this paper to minimize the search space in computing path-label transitive closure. Furthermore, we demonstrate the superiority of our method by extensive experiments.

Due to the increasing use of RDF data, efficient processing of SPARQL queries over RDF datasets has become an important issue. However, existing solutions suffer from two limitations: 1) they cannot answer SPARQL queries with wildcards in a scalable manner; and 2) they cannot handle frequent updates in RDF repositories efficiently. Thus, most of them have to reprocess the dataset from scratch. In this paper, we propose a graph-based approach to store and query RDF data. Rather than mapping RDF triples into a relational database as most existing methods do, we store RDF data as a large graph. A SPARQL query is then converted into a corresponding subgraph matching query. In order to speed up query processing, we develop a novel index, together with some effective pruning rules and efficient search algorithms. Our method can answer exact SPARQL queries and queries with wildcards in a uniform manner. We also propose an effective maintenance algorithm to handle online updates over RDF repositories. Extensive experiments confirm the efficiency and effectiveness of our solution.

Given a record set D and a query score function F, a top-k query returns k records from D, whose values of function F on their attributes are the highest. In this paper, we investigate the intrinsic connection between top-k queries and dominant relationships between records, and based on which, we propose an efficient layer-based indexing structure, Pareto-Based Dominant Graph (DG), to improve the query efficiency. Specifically, DG is built offline to express the dominant relationship between records and top-k query is implemented as a graph traversal problem, i.e., Traveler algorithm. We prove theoretically that the size of search space (that is the number of retrieved records from the record set to answer top-k query) in our algorithm is directly related to the cardinality of skyline points in the record set (see Theorem 3). Considering I/O cost, we propose cluster-based storage schema to reduce I/O cost in Traveler algorithm. We also propose the cost estimation methods in this paper. Based on cost analysis, we propose an optimization technique, pseudorecord, to further improve the search efficiency. In order to handle the top-k query in the high-dimension record set, we also propose N-Way Traveler algorithm. In order to handle DG maintenance efficiently, we propose “Insertion” and “Deletion” algorithms for DG. Finally, extensive experiments demonstrate that our proposed methods have significant improvement over its counterparts, including both classical and state art of top-k algorithms.

The growing popularity of graph databases has generated interesting data management problems, such as subgraph search, shortest-path query, reachability verification, and pattern match. Among these, a pattern match query is more flexible compared to a subgraph search and more informative compared to a shortest-path or reachability query. In this paper, we address pattern match problems over a large data graph G. Specifically, given a pattern graph (i.e., query Q ), we want to find all matches (in G ) that have the similar connections as those in Q. In order to reduce the search space significantly, we first transform the vertices into points in a vector space via graph embedding techniques, coverting a pattern match query into a distance-based multi-way join problem over the converted vector space. We also propose several pruning strategies and a join order selection method to process join processing efficiently. Extensive experiments on both real and synthetic datasets show that our method outperforms existing ones by orders of magnitude.

The growing popularity of social networks has generated interesting data management and data mining problems. An important concern in the release of these data for study is their privacy, since social networks usually contain personal information. Simply removing all identifiable personal information (such as names and social security number) before releasing the data is insufficient. It is easy for an attacker to identify the target by performing different structural queries. In this paper we propose k-automorphism to protect against multiple structural attacks and develop an algorithm (called KM) that ensures k-automorphism. We also discuss an extension of KM to handle "dynamic" releases of the data. Extensive experiments show that the algorithm performs well in terms of protection it provides.

Retrieving related graphs containing a query graph from a large graph database is a key issue in many graph-based applications, such as drug discovery and structural pattern recognition. Because sub-graph isomorphism is a NP-complete problem [4], we have to employ a filter-and-verification framework to speed up the search efficiency, that is, using an effective and efficient pruning strategy to filter out the false positives (graphs that are not possible in the results) as many as possible first, then validating the remaining candidates by subgraph isomorphism checking. In this paper, we propose a novel filtering method, a spectral encoding method, i.e. GCoding. Specifically, we assign a signature to each vertex based on its local structures. Then, we generate a spectral graph code by combining all vertex signatures in a graph. Based on spectral graph codes, we derive a necessary condition for sub-graph isomorphism. Then we propose two pruning rules for sub-graph search problem, and prove that they satisfy the no-false-negative requirement (no dismissal in answers). Since graph codes are in numerical space, we take this advantage and conduct efficient filtering over graph codes. Extensive experiments show that GCoding outperforms existing counterpart methods. 1.

Given a record set D and a query score function F, a top-k query returns k records from D, whose values of function F on their attributes are the highest. In this paper, we investigate the intrinsic connection between top-k queries and dominant relationship between records, and based on which, we propose an efficient layer-based indexing structure, Dominant Graph (DG), to improve the query efficiency. Specifically, DG is built offline to express the dominant relationship between records and top-k query is implemented as a graph traversal problem, i.e. Traveler algorithm. We prove theoretically that the size of search space (that is the number of retrieved records from the record set to answer top-k query) in our basic algorithm is directly related to the cardinality of skyline points in the record set (see Theorem 3.2). Based on the cost analysis, we propose the optimization technique, pseudo record, to improve the search efficiency. In order to handle the top-k query in the high dimension record set, we also propose N-Way Traveler algorithm. Finally, extensive experiments demonstrate that our proposed methods have significant improvement over its counterparts, including both classical and state art of top-k algorithms. For example, the search space in our algorithm is less than 1/5 of that in AppRI (Xin et al., 2006), one of state art of top-k algorithms. Furthermore, our method can support any aggregate monotone query function.