Archive for the 'CBO'

CBO: about the statistical definition of “cardinality” (densities part I)

Let’s explore the concept of cardinality from the point of view of the statistician; this is both to get a clearer vision of the matter (i.e. for fun) and to path the way for understanding the rationale for the “density” statistics as calculated by dbms_stats (the topic of an...
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CBO: the “non-empty result set” assumption

The CBO assumes that SELECT statements are always going to retrieve at least one row – even if this is not necessarily the case, of course. Understanding why this is done is both useful and fascinating. We must start from the very beginning and remember that one of the most important tasks...
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