Abstract
The problem of providing explanation for a query answer is referred to as lineage tracing. This problem has been studied extensively in data warehouse systems, but for mediator-based systems, this is identified as a research problem. In such a system, the mediator does not store data. This means for query processing as well as for tracing, the mediator has to communicate with the data sources. which this communication could be expensive or impossible. so To resolve this, we clearly define forward lineage tracing and show its properties. We propose a tracing method computes data lineage without storing any data and effectively supports aggregation and variable granularity lineage. And we illustrate that our method is more efficient than methods that compute the lineage by executing the reverse query.
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