Query Optimization and Execution

Query optimization refers to the process of translating a declarative query into a query execution plan, i.e., a specific sequence of steps that the query execution engine should follow to evaluate the query. Why is it difficult in data integration systems?

• sources are autonomous; optimizer may not have any statistics or reliability info about the sources, query planning is difficult
• sources are heterogeneous; they may have different query processing capabilities
• data transfer time is not predictable due to the existence of the network environment, it is difficult to make cost estimates
• some sources may become unavailable (?)