Memory disaggregation separates compute (CPU) and main memory resources into disjoint physical units to enable elastic and independent scaling. Connected via high-speed RDMA-enabled networks, compute nodes can directly access remote memory. This setting often requires complex protocols with many network roundtrips as memory nodes have near-zero compute power. In this paper, we design a scalable distributed inverted list index for disaggregated memory architectures. An inverted list index maps a set of terms to lists of documents that contain this term. Current solutions either partition the index horizontally or vertically with severe limitations in the disaggregated memory setting due to data and access skew, high network latency, or out-of-memory errors. Our method partitions lists into fixed-size blocks and spreads them across the memory nodes to balance skewed accesses. Block-based list processing keeps the memory footprint of compute nodes low and masks latency by interleaving remote accesses with expensive list operations. In addition, we propose efficient updates with optimistic concurrency control and read-write conflict detection. Our experiments confirm the efficiency and scalability of our method.

Given two collections of sets, the set similarity join reports all pairs of sets that are within a given distance threshold. State-of-the-art solutions employ an inverted list index and several heuristics to compute the join result efficiently. Prefix-based solutions benefit from infrequent set elements, known as tokens, and spend considerable time scanning long lists if the token frequency is not sufficiently skewed. Partition-based methods are less sensitive to the token distribution but suffer from a significantly larger memory footprint, limiting their applicability as the threshold or the set sizes grow. Solutions from the domain of metric-based similarity search are designed to reduce the overall number of distance computations. Generic metric techniques cannot compete with state-of-the-art similarity joins tailored to sets, which in turn do not exploit metric filter opportunities.We propose MetricJoin, the first exact set similarity join technique that leverages the metric properties of set distance functions. In contrast to its competitors, MetricJoin is robust, i.e., datasets with different characteristics can be joined efficiently in terms of runtime and memory. Our algorithm embeds sets in vector space, organizes long inverted lists in spatial indexes, and employs an effective metric filter to prune unqualified sets. MetricJoin requires only linear space in the collection size and substantially reduces the number of sets that must be considered. In our performance studies, MetricJoin outperforms state-of-the-art solutions by up to an order of magnitude in runtime and generates up to five orders of magnitude fewer candidates.

Density-based clustering aims to find groups of similar objects (i.e., clusters) in a given dataset. Applications include, e.g., process mining and anomaly detection. It comes with two user parameters (ε, MinPts) that determine the clustering result, but are typically unknown in advance. Thus, users need to interactively test various settings until satisfying clusterings are found. However, existing solutions suffer from the following limitations: (a) Ineffective pruning of expensive neighborhood computations. (b) Approximate clustering, where objects are falsely labeled noise. (c) Restricted parameter tuning that is limited to ε whereas MinPts is constant, which reduces the explorable clusterings. (d) Inflexibility in terms of applicable data types and distance functions. We propose FINEX, a linear-space index that overcomes these limitations. Our index provides exact clusterings and can be queried with either of the two parameters. FINEX avoids neighborhood computations where possible and reduces the complexities of the remaining computations by leveraging fundamental properties of density-based clusters. Hence, our solution is efficient and flexible regarding data types and distance functions. Moreover, FINEX respects the original and straightforward notion of density-based clustering. In our experiments on 12 large real-world datasets from various domains, FINEX frequently outperforms state-of-the-art techniques for exact clustering by orders of magnitude.

We study the set similarity join problem , which retrieves all pairs of similar sets from two collections of sets for a given distance function. Existing exact solutions employ a signature-based filter-verification framework: If two sets are similar, they must have at least one signature in common, otherwise they can be pruned safely. We observe that the choice of the signature scheme has a significant impact on the performance. Unfortunately, choosing a good signature scheme is hard because the performance heavily depends on the characteristics of the underlying dataset. To address this problem, we propose a hybrid signature composition that leverages the most selective portion of each signature scheme. Sets with an unselective primary signature are detected, and the signatures are replaced with a more selective secondary signature. We propose a generic framework called TwoL and a cost model to balance the computational overhead and the selectivity of the signature schemes. We implement our framework with two complementary signature schemes for Jaccard similarity and Hamming distance, resulting in effective two-level hybrid indexes that join datasets with diverse characteristics efficiently. TwoL consistently outperforms state-of-the-art set similarity joins on a benchmark with 13 datasets that cover a wide range of data characteristics.

Given a query tree Q, the top-k subtree similarity query retrieves the k subtrees in a large document tree T that are closest to Q in terms of tree edit distance. The classical solution scans the entire document, which is slow. The state-of-the-art approach precomputes an index to reduce the query time. However, the index is large (quadratic in the document size), building the index is expensive, updates are not supported, and data-specific tuning is required.