Graph neural networks (GNNs) are models specialized for graph data and widely used in applications. To train GNNs on large graphs that exceed CPU memory, several systems have been designed to store data on disk and conduct out-of-core processing. However, these systems suffer from either read amplification when conducting random reads for node features that are smaller than a disk page, or degraded model accuracy by treating the graph as disconnected partitions. To close this gap, we build DiskGNN for high I/O efficiency and fast training without model accuracy degradation. The key technique is offline sampling , which decouples graph sampling from model computation . In particular, by conducting graph sampling beforehand for multiple mini-batches, DiskGNN acquires the node features that will be accessed during model computation and conducts pre-processing to pack the node features of each mini-batch contiguously on disk to avoid read amplification for computation. Given the feature access information acquired by offline sampling, DiskGNN also adopts designs including four-level feature store to fully utilize the memory hierarchy of GPU and CPU to cache hot node features and reduce disk access, batched packing to accelerate feature packing during pre-processing, and pipelined training to overlap disk access with other operations. We compare DiskGNN with state-of-the-art out-of-core GNN training systems. The results show that DiskGNN has more than 8x speedup over existing systems while matching their best model accuracy. DiskGNN is open-source at https://github.com/Liu-rj/DiskGNN.

State-of-the-art Text-to-SQL models rely on fine-tuning or few-shot prompting to help LLMs learn from training datasets containing mappings from natural language (NL) queries to SQL statements. Consequently, the quality of the dataset can greatly affect the accuracy of these Text-to-SQL models. Unlike other NL tasks, Text-to-SQL datasets are prone to errors despite extensive manual efforts due to the subtle semantics of SQL. Our study has found a non-negligible (>30%) portion of incorrect NL to SQL mapping cases exists in popular datasets Spider and BIRD. This paper aims to improve the quality of Text-to-SQL training datasets and thereby increase the accuracy of the resulting models. To do so, we propose a necessary correctness condition called execution consistency. For a given database instance, an NL to SQL mapping satisfies execution consistency if the execution result of an NL query matches that of the corresponding SQL. We develop SQLDriller to detect incorrect NL to SQL mappings based on execution consistency in a best-effort manner by crafting database instances that likely result in violations of execution consistency. It generates multiple candidate SQL predictions that differ in their syntax structures. Using a SQL equivalence checker, SQLDriller obtains counterexample database instances that can distinguish non-equivalent candidate SQLs. It then checks the execution consistency of an NL to SQL mapping under this set of counterexamples. The evaluation shows SQLDriller effectively detects and fixes incorrect mappings in the Text-to-SQL dataset, and it improves the model accuracy by up to 13.6%.

Today, the wide adoption of distributed service-oriented applications has rendered multi-database transactions increasingly important. They protect cross-service workflows that access multiple database systems from concurrency anomalies and failures. This paper presents Sonata, a new multi-database transaction system that provides high performance, global serializability, and seamless integration with existing applications and database systems. Sonata builds on the theory of commitment ordering to ensure global serializability and uses two-phase commit for atomicity and durability. Instead of treating database systems as black box storage, Sonata reuses existing database systems' concurrency control yet refrains from exposing or modifying their internals. It performs additional non-blocking coordination only at prepare time via application-level shim layers, allowing applications to incorporate Sonata without changing their existing queries or database systems. Evaluation using TPC-C shows that Sonata incurs 7.1% coordination overhead on average and outperforms prior work by up to 1114.3%.

Today, most database-backed web applications depend on the database to handle deadlocks. At runtime, the database monitors the progress of transaction execution to detect deadlocks and abort affected transactions. However, this common detect-and-recover strategy is costly to performance as aborted transactions waste CPU resources.To avoid deadlock-induced performance degradation, developers aim to reorganize the application code to remove deadlocks. Unfortunately, doing so is difficult for web applications. Not only do their implementations include hundreds of thousands of LoCs, but they also use third-party object-relational mapping (ORM) frameworks which hide database access details. Consequently, it is hard for developers to accurately diagnose deadlocks.We propose WeSEER, a deadlock diagnosis tool for web applications. To overcome the opacity of ORMs, WeSEER performs concolic execution on unit tests to extract a web application’s transactions as a sequence of template statements with symbolic inputs as well as path conditions that enable the sequence. WeSEER then analyzes the extracted transactions based on fine-grained lock modeling to identify potential deadlocks and report the code locations that cause them. We implement WeSEER for Java-based (OpenJDK) web applications, and use it to analyze two popular open-source e-commerce applications, Broadleaf and Shopizer. WeSEER has successfully identified 18 potential deadlocks in Broadleaf and Shopizer. Eliminating these identified deadlocks can result in up to 39.5× and 4.5× throughput improvement for Broadleaf and Shopizer, respectively.

Proving the equivalence between SQL queries is a fundamental problem in database research. Existing solvers model queries using algebraic representations and convert such representations into first-order logic formulas so that query equivalence can be verified by solving a satisfiability problem. The main challenge lies in "unbounded summations", which appear commonly in a query's algebraic representation in order to model common SQL features, such as projection and aggregate functions. Unfortunately, existing solvers handle unbounded summations in an ad-hoc manner based on heuristics or syntax comparison, which severely limits the set of queries that can be supported. This paper develops a new SQL equivalence prover called SQLSolver, which can handle unbounded summations in a principled way. Our key insight is to use the theory of LIA^*, which extends linear integer arithmetic formulas with unbounded sums and provides algorithms to translate a LIA^* formula to a LIA formula that can be decided using existing SMT solvers. We augment the basic LIA^* theory to handle several complex scenarios (such as nested unbounded summations) that arise from modeling real-world queries. We evaluate SQLSolver with 359 equivalent query pairs derived from the SQL rewrite rules in Calcite and Spark SQL. SQLSolver successfully proves 346 pairs of them, which significantly outperforms existing provers.

Distributed transaction systems incur extensive cross-node communication to execute and commit serializable OLTP transactions. As a result, their performance greatly suffers. Caching data at nodes that execute transactions can cut down remote reads. Batching transactions for validation and persistence can amortize the communication cost during committing. However, caching and batching can significantly increase the likelihood of conflicts, causing expensive aborts. In this paper, we develop Hackwrench to address the challenge of caching and batching. Instead of aborting conflicted transactions, Hackwrench tries to repair them using fine-grained re-execution by tracking the dependencies of operations among a batch of transactions. Tracked dependencies allow Hackwrench to selectively invalidate and re-execute only those operations necessary to "fix" the conflict, which is cheaper than aborting and executing an entire batch of transactions. Evaluations using TPC-C and other micro-benchmarks show that Hackwrench can outperform existing commercial and research systems including FoundationDB, Calvin, COCO, and Sundial under comparable settings.

Query rewriting transforms a relational database query into an equivalent but more efficient one, which is crucial for the performance of database-backed applications. Such rewriting relies on pre-specified rewrite rules. In existing systems, these rewrite rules are discovered through manual insights and accumulate slowly over the years. In this paper, we present WeTune, a rule generator that automatically discovers new rewrite rules. Inspired by compiler superoptimization, WeTune enumerates all valid logical query plans up to a certain size and tries to discover equivalent plans that could potentially lead to more efficient rewrites. The core challenge is to determine which set of conditions (aka constraints) allows one to prove the equivalence between a pair of query plans. We address this challenge by enumerating combinations of "interesting" constraints that relate tables and their attributes between each pair of queries. We also propose a new SMT-based verifier to verify the equivalence of a query pair under different enumerated constraints. To evaluate the usefulness of rewrite rules discovered by WeTune, we apply them on the SQL queries collected from the 20 most popular open-source web applications on GitHub. WeTune successfully optimizes 247 queries that existing databases cannot optimize, resulting in substantial performance improvements.

Multicore in-memory databases often rely on traditional con- currency control schemes such as two-phase-locking (2PL) or optimistic concurrency control (OCC). Unfortunately, when the workload exhibits a non-trivial amount of contention, both 2PL and OCC sacrifice much parallel execution op- portunity. In this paper, we describe a new concurrency control scheme, interleaving constrained concurrency con- trol (IC3), which provides serializability while allowing for parallel execution of certain conflicting transactions. IC3 combines the static analysis of the transaction workload with runtime techniques that track and enforce dependencies among concurrent transactions. The use of static analysis simplifies IC3's runtime design, allowing it to scale to many cores. Evaluations on a 64-core machine using the TPC- C benchmark show that IC3 outperforms traditional con- currency control schemes under contention. It achieves the throughput of 434K transactions/sec on the TPC-C bench- mark configured with only one warehouse. It also scales better than several recent concurrent control schemes that also target contended workloads.

The majority of people in rural developing regions do not have access to the World Wide Web. Traditional network connectivity technologies have proven to be prohibitively expensive in these areas. The emergence of new long-range wireless technologies provide hope for connecting these rural regions to the Internet. However, the network connectivity provided by these new solutions are by nature intermittent due to high network usage rates, frequent power-cuts and the use of delay tolerant links. Typical applications, especially interactive applications like web search, do not tolerate intermittent connectivity. In this paper, we present the design and implementation of RuralCafe, a system intended to support efficient web search over intermittent networks. RuralCafe enables users to perform web search asynchronously and find what they are looking for in one round of intermittency as opposed to multiple rounds of search/downloads. RuralCafe does this by providing an expanded search query interface which allows a user to specify additional query terms to maximize the utility of the results returned by a search query. Given knowledge of the limited available network resources, RuralCafe performs optimizations to prefetch pages to best satisfy a search query based on a user's search preferences. In addition, RuralCafe does not require modifications to the web browser, and can provide single round search results tailored to various types of networks and economic constraints. We have implemented and evaluated the effectiveness of RuralCafe using queries from logs made to a large search engine, queries made by users in an intermittent setting, and live queries from a small testbed deployment. We have also deployed a prototype of RuralCafe in Kerala, India.