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1. IcedTea: Efficient and Responsive Time-Travel Debugging in Dataflow Systems

   https://doi.org/10.14778/3712221.3712251  

   Ni, Shengquan; Huang, Yicong; Wang, Zuozhi; Li, Chen (September 2025, Proceedings of the VLDB Endowment)

   Dataflow systems have an increasing need to support a wide range of tasks in data-centric applications using latest techniques such as machine learning. These tasks often involve custom functions with complex internal states. Consequently, users need enhanced debugging support to understand runtime behaviors and investigate internal states of dataflows. Traditional forward debuggers allow users to follow the chronological order of operations in an execution. Therefore, a user cannot easily identify a past runtime behavior after an unexpected result is produced. In this paper, we present a novel time-travel debugging paradigm called IcedTea, which supports reverse debugging. In particular, in a dataflow's execution, which is inherently distributed across multiple operators, the user can periodically interact with the job and retrieve the global states of the operators. After the execution, the system allows the user to roll back the dataflow state to any past interactions. The user can use step instructions to repeat the past execution to understand how data was processed in the original execution. We give a full specification of this powerful paradigm, study how to reduce its runtime overhead and develop techniques to support debugging instructions responsively. Our experiments on real-world datasets and workflows show that IcedTea can support responsive time-travel debugging with low time and space overhead. 

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2. The Key to Effective UDF Optimization: Before Inlining, First Perform Outlining

   https://doi.org/10.14778/3696435.3696436  

   Arch, Samuel; Liu, Yuchen; Mowry, Todd C; Patel, Jignesh M; Pavlo, Andrew (September 2024, Proceedings of the VLDB Endowment)

   Although user-defined functions (UDFs) are a popular way to augment SQL's declarative approach with procedural code, the mismatch between programming paradigms creates a fundamental optimization challenge. UDF inlining automatically removes all UDF calls by replacing them with equivalent SQL subqueries. Although inlining leaves queries entirely in SQL (resulting in large performance gains), we observe that inlining the entire UDF often leads to sub-optimal performance. A better approach is to analyze the UDF, deconstruct it into smaller pieces, and inline only the pieces that help query optimization. To achieve this, we propose UDF outlining, a technique to intentionally hide pieces of a UDF from the optimizer, resulting in simpler UDFs and significantly faster query plans. Our implementation (PRISM) demonstrates that UDF outlining improves performance over conventional inlining (on average 1.29× speedup for DuckDB and 298.73× for SQL Server) through a combination of more effective unnesting, improved data skipping, and by avoiding unnecessary joins. 

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3. Lightweight Materialization for Fast Dashboards Over Joins

   https://doi.org/10.1145/3626735  

   Huang, Zezhou; Wu, Eugene (December 2023, Proceedings of the ACM on Management of Data)

   Dashboards are vital in modern business intelligence tools, providing non-technical users with an interface to access comprehensive business data. With the rise of cloud technology, there is an increased number of data sources to provide enriched contexts for various analytical tasks, leading to a demand for interactive dashboards over a large number of joins. Nevertheless, joins are among the most expensive operations in DBMSes, making the support of interactive dashboards over joins challenging. In this paper, we present Treant, a dashboard accelerator for queries over large joins. Treant uses factorized query execution to handle aggregation queries over large joins, which alone is still insufficient for interactive speeds. To address this, we exploit the incremental nature of user interactions using Calibrated Junction Hypertree (CJT), a novel data structure that applies lightweight materialization of the intermediates during factorized execution. CJT ensures that the work needed to compute a query is proportional to how different it is from the previous query, rather than the overall complexity. Treant manages CJTs to share work between queries and performs materialization offline or during user think-times. Implemented as a middleware that rewrites SQL, Treant is portable to any SQL-based DBMS. Our experiments on single node and cloud DBMSes show that Treant improves dashboard interactions by two orders of magnitude, and provides 10x improvement for ML augmentation compared to SOTA factorized ML system. 

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4. Insights from Student Solutions to SQL Homework Problems

   https://doi.org/10.1145/3341525.3387391  

   Poulsen, Seth; Butler, Liia; Alawini, Abdussalam; Herman, Geoffrey L. (June 2020, Proceedings of the 2020 ACM Conference on Innovation and Technology in Computer Science Education)

   null (Ed.)

   We analyze the submissions of 286 students as they solved Structured Query Language (SQL) homework assignments for an upper-level databases course. Databases and the ability to query them are becoming increasingly essential for not only computer scientists but also business professionals, scientists, and anyone who needs to make data-driven decisions. Despite the increasing importance of SQL and databases, little research has documented student difficulties in learning SQL. We replicate and extend prior studies of students' difficulties with learning SQL. Students worked on and submitted their homework through an online learning management system with support for autograding of code. Students received immediate feedback on the correctness of their solutions and had approximately a week to finish writing eight to ten queries. We categorized student submissions by the type of error, or lack thereof, that students made, and whether the student was eventually able to construct a correct query. Like prior work, we find that the majority of student mistakes are syntax errors. In contrast with the conclusions of prior work, we find that some students are never able to resolve these syntax errors to create valid queries. Additionally, we find that students struggle the most when they need to write SQL queries related to GROUP BY and correlated subqueries. We suggest implications for instruction and future research. 

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5. ROME: Robust Query Optimization via Parallel Multi-Plan Execution

   https://doi.org/10.1145/3654973  

   Wei, Ziyun; Trummer, Immanuel (May 2024, Proceedings of the ACM on Management of Data)

   We present a non-intrusive approach to robust query processing that can be used on top of any SQL execution engine. To reduce the risk of selecting highly sub-optimal query plans, we execute multiple plans in parallel. Query processing finishes once the first of these plans finishes execution. Plans are selected to be complementary in terms of the intermediate results they generate. This increases robustness to cardinality estimation errors, making cost prediction hard, that concern a subset of candidate results. We present multiple cost-based approaches to selecting plans for robust execution. The first approach uses a simple cost model, based on diversity of intermediate results. The second approach features a probabilistic model, approximating expected execution overheads, given uncertainty on true intermediate result sizes. We present greedy and exhaustive algorithms to select optimal plans according to those cost models. The experiments demonstrate that executing multiple plans in parallel is preferable over executing single plans that are occasionally sub-optimal, as well as over several baselines. 

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