Optimization and Execution of Complex Scientific Queries
Sammanfattning: Large volumes of data produced and shared within scientific communities are analyzed by many researchers to investigate different scientific theories. Currently the analyses are implemented in traditional programming languages such as C++. This is inefficient for research productivity, since it is difficult to write, understand, and modify such programs. Furthermore, programs should scale over large data volumes and analysis complexity, which further complicates code development.This Thesis investigates the use of database technologies to implement scientific applications, in which data are complex objects describing measurements of independent events and the analyses are selections of events by applying conjunctions of complex numerical filters on each object separately. An example of such an application is analyses for the presence of Higgs bosons in collision events produced by the ATLAS experiment. For efficient implementation of such an ATLAS application, a new data stream management system SQISLE is developed. In SQISLE queries are specified over complex objects which are efficiently streamed from sources through the query engine. This streaming approach is compared with the conventional approach to load events into a database before querying. Since the queries implementing scientific analyses are large and complex, novel techniques are developed for efficient query processing. To obtain efficient plans for such queries SQISLE implements runtime query optimization strategies, which during query execution collect runtime statistics for a query, reoptimize the query using the collected statistics, and dynamically switch optimization strategies. The cost-based optimization utilizes a novel cost model for aggregate functions over nested subqueries. To alleviate estimation errors in large queries the fragments are decomposed into conjunctions of subqueries over which runtime statistics are measured. Performance is further improved by query transformation, view materialization, and partial evaluation. ATLAS queries in SQISLE using these query processing techniques perform close to or better than hard-coded C++ implementations of the same analyses.Scientific data are often stored in Grids, which manage both storage and computational resources. This Thesis includes a framework POQSEC that utilizes Grid resources to scale scientific queries over large data volumes by parallelizing the queries and shipping the data management system itself, e.g. SQISLE, to Grid computational nodes for the parallel query execution.
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