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Basic Concept

原创 Linux操作系统 作者:佩奇 时间:2009-10-04 22:49:15 0 删除 编辑

Selectivity: a value between 0 and 1 representing the fraction of rows filtered by an operation....
  e.g.: an access operation reads 120 rows from a table and after applying a filter returns 18 of them, then the selecitvity is 0.15(18/120)

Cardinality: the number of rows returned by an operation, in last example is 18

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