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Query optimization in distributed databases explicitly needed in many aspects of the optimization process, often making it imperative for the optimizer to consult underlying data sources while doing cost based optimization. This is not only increases the cost of optimization, but also changes the trade-offs involved in the optimization process significantly. The leading cost in this optimization process is the "Cost of costing" that traditionally has been considered insignificant. The optimizer can only afford a few rounds of messages to the under-lying data sources and hence the optimization techniques in this environment must be geared toward gathering all the required cost information with minimal communication.
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