selectivity index 选择指数
index of selectivity 选择性指数
SI Selectivity Index 选择性指数
elignification selectivity index 脱木素选择性指数
selectivity of index 索引的选择性
GSI geometric selectivity index 几何选择性指数
For comparison, Figure 3 illustrates the access plan where I forced the DB2 optimizer to ignore the index by adding the clause SELECTIVITY 1 to the predicate in the where clause.
为了便于比较，图3显示了通过将SELECTIVITY 1子句添加到where子句的谓词中，强迫db2优化器忽略索引的访问计划。
Indexes on the foreign keys referencing tables with few rows have low selectivity, and the utility of requiring the index has always been questionable.
外键上的索引所引用的表行数较少，选择性较少，需要索引的实用程序一直遭受质疑。
Selectivity: When the query optimizer (QO) studies your SQL, the degree of selectivity determines if an index should be used to perform the operation.
选择性：当查询优化器(query optimizer，即QO)读取SQL时，选择性的程度决定了索引是否应该用来执行该操作。
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