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Recent researches in distributed query processing assume that CPU is relatively cheap resource, while network IO and disk IO are equally costly operations. Partition pruning not only saves network requests, but also avoids unnecessary executionы execution of local query parts, preventing page cache trashing and additional disk reads (mostly random). 

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WHERE conditions and JOIN-s are relatively simple targets for extract partition extraction. The main difficulty is subqueries, as they potentially introduce unlimited nesting what that complicates analysis a lot. So the main goal of query rewrite is to minimize number of subqueries. Main techniques are JOIN conversion, correlation and materialization. Interestingly, all these techniques not only simplify partition extraction, but also allow optimizer to choose better execution plan for local execution, as JOINs are much easier to be analyzed and reordered than subqueries. As we shall see below, efforts are usually focused on non-correlated subqueries as they are the most costly for straightforward execution.

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