Wednesday, November 08, 2006

Why Semijoin is Better than Join

Query Processing in a System for Distributed Databases (SDD-1) "We prefer semijoins to joins for three reasons. First, Ri(A = B) Rj subsumes Ri, and so semijoins monotonically reduce the size of the database. By contrast, joins can increase the size of the database; in the worst case...Second, semijoins can be computed with less intersite data transfer than joins...we need only transmit a projection of a relation...the third advantage of semijoins is that the “reductive effect” of any single join can be attained by two semijoins, usually at lower cost..."

This is one of the first times, in many years, where I had to resort to using another search engine other than Google (Yahoo) to find this paper.
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